DETAILED DESCRIPTION OF THE INVENTION

[0046] Referring now to the accompanying drawings (FIGS. 1 to 11 ), there is shown one preferred embodiment of the invention.

[0047] (General Configuration)

[0048] FIG. 1 shows a general configuration of a monitoring system 10 using a fully wireless communication system of on-demand connection type (communication system) of the embodiment. The monitoring system 10 includes a master controller 11 , a communication system 12 , and a sensor 13 . The communication system 12 includes a master 12 a and a slave 12 b . The master controller 11 is connected to the master 12 a , the sensor 13 is connected to the slave 12 b , and radio communications are conducted between the master 12 a and the slave 12 b.

[0049] The monitoring system 10 is an example of a system using the communication system 12 and the communication system of the invention can also be applied to other systems.

[0050] FIG. 1 shows only a single slave 12 a and a single sensor 13 , but it is assumed that the monitoring system 10 includes a plurality of slaves 12 b and sensors 13 for a pair of the master controller 11 and the master 12 a . The numbers of the slaves 12 b and the sensors 13 may be each one.

[0051] In FIG. 1 , the dashed line indicates a feeder for supplying power, the alternate long and short dash line indicates a feed start line, and the solid line indicates a circuit-to-circuit connection line (bus, etc.,).

[0052] The monitoring system 10 monitors an anomaly by the sensor 13 . When the sensor 13 detects an anomaly the monitoring system 10 reports occurrence of the anomaly to the master controller 11 via the slave 12 b and the master 12 a . The monitoring system 10 sends a control command from the master controller 11 via the master 12 a and the slave 12 b to the sensor 13 .

[0053] The master 12 a includes a radio communication section 21 (first communication section), a CPU (central processing unit) 22 , memory 23 , and a power supply section 24 .

[0054] The radio communication section 21 transmits and receives a packet in radio communications with the slave 12 b and also transmits a start signal to the slave 12 b as described later. The CPU 22 controls communications of the radio communication section 21 and controls data transfer to and from the master controller 11 , etc. The memory 23 stores data and programs required for processing of the CPU 22 ; for example, the memory 23 stores a communication protocol used for communications of the radio communication section 21 . The power supply section 24 acquires electric power required for the master controller 11 and the master 12 a from commercial power supply and supplies the power to the sections. When the communication system 12 operates, the power supply section 24 basically supplies power to the sections at all times.

[0055] The slave 12 b includes a radio communication section 31 (second communication section), a CPU 32 , a battery unit 34 (which may be a storage battery unit), a power supply control section 35 , a timer 36 , and a sensor I/F (interface) 37 .

[0056] The radio communication section 31 transmits and receives a packet in radio communications with the master 12 a and also receives a start signal from the master 12 a . The CPU 32 controls communications of the radio communication section 31 and controls data transfer to and from the sensor 13 through the sensor I/F 37 , etc. The memory 33 stores data and programs required for processing of the CPU 32 ; for example, the memory 33 stores a communication protocol used for communications of the radio communication section 31 .

[0057] The slave 12 b operates with the internal battery unit 34 . Therefore, an external feeder is not connected to the slave 12 b . Since the slave 12 b conducts radio communications with the master 12 a , an external communication line is not connected to the slave 12 b either. Thus, the slave 12 b is fully wireless and the communication system 12 is a fully wireless communication system. The master 12 a acquires electric power from commercial power supply and thus has a feeder. In the specification, however, the communication system including the fully wireless slave 12 b , namely, the slave 12 b to which a feeder, a communication line with the master 12 a , and the like are not connected is called “fully wireless communication system.”

[0058] The battery unit 34 includes a main power supply 34 a and a standby power supply 34 b . The main power supply 34 a supplies power for operating the radio communication section 31 , the CPU 32 , and the memory 33 . The main power supply 34 a is turned on and off under the control of the power supply control section 35 , thereby supplying and stopping power to the radio communication section 31 , the CPU 32 , and the memory 33 . This means that switching is performed between supplying and stopping power to the radio communication section 31 , the CPU 32 , and the memory 33 appropriately in response to the operation state of the slave 12 b under the control of the power supply control section 35 .

[0059] The sensor 13 is a power-thrifty sensor containing a unique power supply such as a battery and can operate independently of power supply from the main power supply 34 a . The sensor I/F 37 can also operate independently of power supply from the main power supply 34 a.

[0060] On the other hand, when the slave 12 b operates, power from the standby power supply 34 b is supplied to the timer 36 at all times. Therefore, the timer 36 always operates regardless of the operation state of the slave 12 b unless the slave 12 b is in a stop state.

[0061] The timer 36 performs count operation to switch the slave 12 b from a standby state to a communication control state at a predetermined timing and to switch the slave 12 b from the communication control state to the standby state at a predetermined timing as described later. The timer 36 times-out at the predetermined timing and as the timer 36 times-out, it gives a power supply start instruction or a power supply stop instruction to the power supply control section 35 .

[0062] The sensor I/F 37 functions as an interface with the sensor 13 and when the sensor 13 detects an anomaly, the sensor I/F 37 gives a power supply start instruction to the power supply control section 35 to report the anomaly to the master controller 11 .

[0063] The power supply control section 35 may be able to turn on/off the main power supply 34 as the user operates an external switch placed on the slave 12 b or the sensor 13 (not shown)

[0064] (State Transition)

[0065] FIG. 2 is a state transition diagram of the master 12 a and the slave 12 b . The master 12 a can take a stop state (STOP state) and a communication state (COM state) The slave 12 b can take a stop state (STOP state), a standby state (SBB state) a communication link control state (communication control state, SCAN state), and a communication state (start state, COM state) The states and the correlation between the states will be discussed below:

[0066] To begin with, the master 12 a will be discussed. The stop state is a state in which the power supply section 24 is off and the master 12 a does not function. When the user, etc., turns on the power supply section 24 , the master 12 a makes a transition to the communication state and enters a state in which the master 12 a can communicate with the slave 12 b . In the communication state of the master 12 a , corresponding to both the communication state and the communication link control state of the slave 12 b described later, a polling packet is transmitted to the slave 12 b and a reply packet is received from the slave 12 b , whereby communications with the slave 12 b and communication link control are performed. When the user, etc., turns off the power supply section 24 , the master 12 a makes a transition to the stop state.

[0067] Next, the slave 12 b will be discussed. The stop state is a state in which the main power supply 34 a and the standby power supply 34 b are off and the slave 12 b does not function. When the user, etc., turns on the standby power supply 34 b , the slave 12 b makes a transition to the standby state and the timer 36 is started. In the standby state, the main power supply 34 a is off and power supply to the radio communication section 31 , the CPU 32 , and the memory 33 is stopped. In the description that follows, the power supply control section 35 turning off the main power supply 34 a is the power supply control section 35 “off” and the power supply control section 35 turning on the main power supply 34 a is the power supply control section 35 “on.”

[0068] The standby state is a time limit state and when the timer 36 times-out, the power supply control section 35 is turned on and the slave 12 b makes a transition to the communication link control state. In the standby state; when-the sensor I/F 37 makes a communication start request (start request), the power supply control section 35 is also turned on and the slave 12 b makes a transition to the communication link-control state.

[0069] The standby state is a state in which power is less consumed than in the communication state or the communication link control state; it is a state of power consumption of about 200 μA, for example.

[0070] The communication link control state is a state in which the slave 12 b can communicate with the master 12 a . However, the communication link control state differs from the communication state in that it is a time limit state and basically the slave 12 b makes a transition to the standby state when the timer 36 times-out or under the control of the CPU 32 .

[0071] If the slave 12 b makes a transition to the communication link control state as the timer 36 times-out in the standby state, when the radio communication section 31 receives a start signal from the slave 12 a in the communication link control state, the count operation of the timer 36 is stopped and the time limit state is released and thus the slave 12 b makes a transition to the communication state.

[0072] If the slave 12 b makes a transition to the communication link control state as the sensor I/F 37 makes a communication start request in the standby state, when a start signal is transmitted to the slave 12 a and a response to the start signal is detected in the communication link control state, the count operation of the timer 36 is stopped and the time limit state is released and thus the slave 12 b makes a transition to the communication state.

[0073] Here, the communication link control state is defined as a state in which the slave 12 b can communicate with the master 12 a as in the communication state. However, the communication link control state need not necessarily be a state in which the slave 12 b can completely communicate with the master 12 a and may be at least a state in which the slave 12 b can receive a start signal transmitted by the master 12 a and can make a transition to the communication state upon reception of the start signal.

[0074] The communication state is a state in which the slave 12 b can communicate with the master 12 a . In the communication state unlike the communication link control state, as communication termination processing is performed according to the communication protocol in communications with the master 12 a , the slave 12 b makes a transition to the communication link control state and opens the communication link with the master 12 a . The communication termination processing is based on an instruction from the master 12 a or an instruction through a high-level protocol (communication high-level protocol) from the sensor I/F 37 . Therefore, the communication state is continued until completion of communications-with the mater 12 a.

[0075] In the communication state, a communication packet from the master 12 a is superposed on a polling packet from the master 12 a . A communication packet to the master 12 a is transmitted as a reply packet to the polling packet from the master 12 a.

[0076] As described above, the slave 12 b can take any of the communication start (start state) in which the slave 12 b can communicate with the master 12 a , the communication link control state (communication control state) in which at least the slave 12 b can receive a start signal (start request signal) transmitted by the master 12 a and makes a transition to the communication state upon reception of the start signal, or the standby state in which power is less consumed than in the communication state or the communication link control state.

[0077] The slave 12 b can repeatedly operate so that the slave 12 b in the standby state is switched to the communication link control state at a predetermined timing and is switched to the standby state when the communication link control state continues for a predetermined time period without receiving a start signal from the master 12 a . The repeated operation is executed by the timer 36 , the power supply control section 35 , and the CPU 32 , which make up a state control section.

[0078] When the slave 12 b and the master 12 a do not communicate with each other, basically the slave 12 b is in the standby state and power consumption in the slave 12 b can be decreased. The slave 12 b , which is in the standby state, is switched to the communication link control state in a predetermined period under the control of the state control section and monitors a start signal from the master 12 a . Thus, if it becomes necessary for the master 12 a to communicate with the slave. 12 b , the master 12 a can also transmit a start signal at an independent timing of the slave 12 b for causing the slave 12 b to switch to the communication state for conducting communications therebetween.

[0079] The master 12 a basically is in the communication state at all times and thus can receive a communication request from the slave 12 b at any time.

[0080] Therefore, the communication system 12 makes it possible to start the slave 12 b upon reception of a communication start request from the master 12 a while decreasing power consumption in the slave 12 b . That is, the power-thrifty slave 12 b that can also be used for long hours with the battery unit 34 while enabling the master 12 a and the slave 12 b to each make a communication start request can be provided.

[0081] (Communication Scheme 1 )

[0082] FIG. 3 shows the packet transmission/reception state in communications between the master 12 a and the slave 12 b . It is assumed that the communication system 12 includes slaves 1 , 2 , and 3 as the slaves 12 b and that the master 12 a communicates with the slaves 1 , 2 , and 3 according to polling from the master 12 a . The polling period is the period in which the master 12 a polls the slaves 1 , 2 , and 3 in order.

[0083] The slaves 1 and 3 , which are already in the communication state (COM state), receive a communication packet as a polling packet (POLL-PKT) from the master 12 a and transmit each a reply packet to the master 12 a.

[0084] The slave 2 initially is in the standby state (SBY state) In the standby state, even if a start signal is transmitted by polling from the master 12 a , the start signal cannot be detected and a response signal cannot be transmitted either.

[0085] However, the slave 2 makes a transition to the communication link control state (SCAN state) at a predetermined timing independent of the master 12 a . When the slave 2 is in the communication link control state, if the master 12 a transmits a start signal to the slave 2 , the slave 2 can detect the start signal and transmits a response signal. Accordingly, the slave 2 makes a transition to the communication state and from the next polling period, the slave 2 receives a communication packet from the master 12 a and transmits a response packet.

[0086] The slave 2 can remain in the communication link control state only up to α polling periods at the maximum and makes a transition to the standby state when the α polling periods are exceeded. α is an adequate numeral of two or more and is set as an initial value.

[0087] The master 12 a polls all slaves 1 to 3 regardless of the state of each slave. To poll the slaves, the master 12 a transmits a communication packet (communication PKT) to the slave in the communication state and transmits a start signal to the slave in any state other than the communication state upon reception of a communication start request or a null packet (NULL-PKT) upon reception of no communication start request.

[0088] When the master 12 a polls each slave and does not receive a reply packet (reply PKT) from the slave, the following processing is performed in response to the state of the slave. If a reply packet is not returned within a given time from the polled slave in the communication state, it is determined that the slave is in an abnormal condition of failure, etc.

[0089] On the other hand, to transmit a start signal in polling the slave in any state other than the communication state, the start signal is transmitted in two polling periods at the minimum and a response signal as a reply packet is not returned within a given time, it is determined that the slave is in an abnormal condition of failure, etc. To transmit a null packet in polling the slave in any state other than the communication state, if a reply packet is not returned, it is not determined that the slave is in an abnormal condition, and usual processing is continued.

[0090] Thus, the master 12 a switches the assignment time periods of communications with the slaves 1 to 3 in order, thereby communicating with the slaves 1 to 3 and- to transmit a start signal, the master 12 a transmits the start signal in the assignment time period of communications with the slave to which the start signal is transmitted. Therefore, the master 12 a transmits the start signal in the assignment time period of communications with the target slave as a part of polling processing.

[0091] When the slave 12 b makes a communication start request, the packet transmission/reception state in communications between the master 12 a and the slave 12 b becomes as shown in FIG. 4 .

[0092] For example, if the slave 2 makes a communication start request, the slave 2 receives a null packet sent from the master 12 a and transmits a start signal to the master 12 a as a response packet. The master 12 a transmits a response signal to the start signal to the slave 2 in the next polling period and the slave 2 detects the response signal and accordingly makes a transition from the communication link control state to the communication state.

[0093] (Communication Scheme 2 )

[0094] In the scheme in FIG. 3 , the start signal is transmitted from the master 12 a to the slave 12 b when the master 12 a polls the slave 12 b . On the other hand, the slave 12 b makes a transition from the standby state to the communication link control state in the period independent of the polling period and is polled by the master 12 a . Thus, in the scheme in FIG. 3 , the start signal from the master 12 a is not detected until the next polling period depending on the transition timing of the slave 12 b from the standby state to the communication link control state.

[0095] This causes a time lag (start signal detection delay) to occur in the slave 12 b in detecting a communication start request from the master 12 a.

[0096] Further, in the scheme in FIG. 3 , for the slave 12 b to detect the start signal from the master 12 a in the next polling period at the latest, the duration of the standby state of the slave 12 b needs to be set less than the polling period and the duration of the communication link control state of the slave 12 b needs to be set more than the polling period. Therefore, it becomes difficult to make sufficiently small the ratio of the duration of the communication link control state to the duration of the standby state, and the power consumption decrease effect in the slave 12 b is somewhat small. Particularly, if the polling period is prolonged with an increase in the number of the slaves 12 b and the duration of the standby state is shortened to reduce the start signal detection delay in the slave 12 b , the power consumption decrease effect more is reduced.

[0097] To more reliably detect the start signal from the master 12 a in the next polling period at the latest considering the time required for the slave 12 b to make a transition from the standby state to the communication link control state (the time until it is made possible for the slave 12 b to actually operate in the communication link control state, arising from the machine operation, the software period, etc.,), it is also considered that the duration of the communication link control state of the slave 12 b is set to twice or more the polling period, in which case the power consumption decrease effect furthermore is reduced.

[0098] To remedy this point, it is desirable that a scheme shown in FIG. 5 should be adopted. In this scheme, the master 12 a transmits a start signal as a polling packet to be transmitted to all slaves at the same time (broadcast polling packet) rather than a polling packet to be transmitted to each slave individually (party specification polling packet). Transmission of the broadcast polling packet is started immediately when the master 12 a sends a communication start request to any of the slaves 1 to 3 , and the broadcast polling packet is repeatedly transmitted until a response signal from the slave 12 b to which the communication start request is sent is acknowledged. Therefore, transmission of the broadcast polling packet is repeated at the maximum for the time resulting from adding a predetermined margin to the duration of the standby state in the slave 12 b . This margin is set considering the time required for the slave 12 b to make a transition from the standby state to the communication link control state (the time until it is made possible for the slave 12 b to actually operate in the communication link control state, arising from the machine operation, the software period, etc.,).

[0099] Thus, to transmit the start signal, it is desirable that the master 12 a should repeatedly transmit the start signal for a longer time than the time required until, after making a transition to the communication link control state, the slave 12 b makes a transition to the standby state and further to the communication link control state. In this case, when the slave 12 b makes a transition to the communication link control state first after the master 12 a starts to transmit the start signal, the master 12 a still repeats transmission of the start signal. Accordingly, when the slave 12 b makes a transition to the communication link control state first after the master 12 a starts to transmit the start signal, it is made possible for the slave 12 b to make a transition to the communication state. Therefore, if the master 12 a makes a start request, it is made possible to cause the slave 12 b to make a transition to the start state as quickly as possible.

[0100] To also transmit a party specification polling packet as a usual communication packet, the master 12 a transmits the broadcast polling packet meanwhile between one timing at which the party specification polling packet is transmitted and another timing.

[0101] Accordingly, the start signal detection delay in the slave 12 b can be decreased. The ratio of the duration of the communication link control state to the duration of the standby state in the slave 12 b can be sufficiently reduced, and the power consumption in the slave 12 b can be decreased drastically.

[0102] Referring to FIGS. 6A and 6B , a comparison is made between the schemes in FIGS. 3 and 5 with respect to the power consumption decrease effect. Here, it is assumed that the power consumption in the standby state is 200 μA and the power consumption in the communication link control state is 50 mA.

[0103] As shown in FIG. 6 A, the traffic model in the scheme in FIG. 3 is as follows: Polling period Tp is one second, the period in which a transition from the standby state to the communication link control state occurs in the slave 12 b (transition period) is three seconds, and duration of the communication link control state of the slave 12 b (worst value of the time required for detecting start signal from the master 12 a after transition to the communication link control state (delay time)) Ts is one second.

[0104] At this time, the average power consumption of the slave 12 b not in the communication state becomes

(200 μA ×(3−1) sec+ 50 mA× 1 sec )÷3 sec =16.8 mA

[0105] On the other hand, as shown in FIG. 6 B, the traffic model in the scheme in FIG. 5 is as follows: Polling period Tp is one second, the transition period is three seconds, and duration of the communication link control state of the slave 12 b Ts is 0.1 sec.

[0106] The reason why Ts=0.1 sec is as follows: The duration of the communication link control state of the slave 12 b needs to be set to a time for allowing another start signal transmitted from the master 12 a to be received if transmission of a party specification polling packet from the master 12 a happens to overlap the duration of the communication link control state. Assuming that the packet length of the party specification polling packet is 2 KB (kilobytes) and the transmission speed is 1 Mbps (megabits per second), the time during which a start signal cannot be transmitted as a party specification polling packet is transmitted becomes

(2000×8)÷1000000=16 millisec

[0107] Therefore, as the duration of the communication link control state of the slave 12 b Ts is set to 0.1 sec, if transmission of a party specification polling packet from the master 12 a happens to overlap the duration of the communication link control state, another start signal transmitted from the master 12 a can be received sufficiently. Therefore, Ts is set to 0.1.

[0108] Thus, it is desirable that the time period until the slave 12 b is switched to the standby state after switched to the communication link control state should be a sufficiently long time period (for example, time period of duration about six times the assignment period) so as to be able to include one assignment time period to transmission of a party specification polling packet in the master 12 a and the time required for transmitting a start signal. Accordingly, when the slave 12 b makes a transition to the communication link control state, if the master 12 a is in the assignment time period to transmission of a party specification polling packet, the slave 12 b is still in the communication link control state when the assignment time period terminates and a start signal is transmitted. Therefore, failing to receive a start signal in the slave 12 b can be circumvented and the slave 12 b can be started immediately in response to a start request made by the master 12 a.

[0109] At this time, the average power consumption of the slave 12 b not in the communication state becomes

(200 μA ×(3−0.1) sec+ 50 mA× 0.1 sec )÷3 sec= 1.86 mA

[0110] Thus, in the scheme in FIG. 5 in contrast to that in FIG. 3 , the duration of the standby state can be set to a sufficiently long time period relative to the time period between the slave 12 b making a transition to the communication link control state and making a transition to the standby state. It is desirable that the duration of the standby state should be 20 times or more the time period between the slave 12 b making a transition to the communication link control state and making a transition to the standby state. For example, as described above, the time period between the slave 12 b making a transition to the communication link control state and making a transition to the standby state is 0.1 sec and the duration of the standby state is 3−0.1=2.9 sec. Accordingly, the ratio of the time period in which the salve 12 b is in the communication link control state can be made sufficiently small and the average power consumption of the slave 12 b not in the communication state can be decreased drastically.

[0111] Thus, the master 12 a communicates with the slaves 1 to 3 according to the assignment order of communications with the slaves 1 to 3 and meanwhile when it becomes necessary for the master 12 a to transmit a start signal, immediately the master 12 a starts to transmit the start signal and repeats start signal transmission. While start signal transmission is repeated, if the transmission timing of a party specification polling packet from the master 12 a to the slave 12 b happens to overlap the start signal transmission, the party specification polling packet and the start signal are transmitted alternately, whereby the transmission delay of the party specification polling packet can be reduced. Accordingly, if the number of the slaves 12 b in the communication state increases and the number of party specification polling packets grows, the transmission delay of the start signal can be reduced. When there is no party specification polling packet transmitted from the master 12 a , only the start signal is transmitted repeatedly, in which case the communication line is idle and therefore a problem does not arise.

[0112] As described above, in the scheme in FIG. 5 , the master 12 a switches the assignment time periods of communications with the slaves 1 to 3 in order, thereby communicating with the slaves 1 to 3 and to transmit a start signal, the master 12 a transmits the start signal using the time period between the assignment time periods of communications with the slaves 1 to 3 . Accordingly, if the number of slaves grows, enlarging of the start signal transmission timing interval can be suppressed. Consequently, while the duration of the communication link control state in the slave is set to a shorter time period, the slave can receive the start signal promptly and can respond to a communication start request from the mater 12 a promptly.

[0113] The start signal transmitted as a broadcast polling packet may be intended for one slave, but may be intended for two or more slaves so that it can cause the two or more slaves to make a transition to the communication state at the same time. In this case, when the master causes the two or more slaves to make a transition to the start state, it can cause the two or more slaves to make a transition to the start state by a common start request signal. Whether the start signal is intended for one slave or two or more slaves can be determined by a communication protocol between the master 12 a and the slaves 12 b , for example.

[0114] (Operation Flow of Slave)

[0115] FIG. 7 shows an operation flow of the slave 12 b in the standby state. In the standby state, the timer 36 reads count T (S 11 ) and determines whether or not T is 0 (S 12 ). If T is not 0, the timer 36 decrements the count T (S 13 ) and if the sensor I/F 37 does not make a communication start request (S 14 , S 15 ), control returns to step S 11 .

[0116] If T is 0 at step S 12 , namely, a timeout occurs or if the sensor I/F 37 makes a communication start request at step S 15 , the timer 36 sets the count T to default value N (S 16 ) and turns on the power supply control section 35 and the slave 12 b makes a transition to the communication link control state (S 7 ).

[0117] FIG. 8 shows an operation flow of the slave 12 b in the communication link control state. Here, it is assumed that to transit a start signal to the master 12 a , the slave 12 b remains in the communication link control state for the time as long as a polling periods to wait for a response signal from the master 12 a after transmitting the start signal. This operation flow basically is executed by the CPU 32 functioning as the state control section for executing a communication link control program stored in the memory 33 . Steps 22 , 24 and 29 in FIG. 8 are executed by the radio communication section 31 .

[0118] First, a polling period counter is set to 1 (S 21 ). When the master 12 a transmits a polling packet to the slave 12 b , the slave 12 b receives the polling packet (S 22 ). If the received polling packet is a start signal (S 23 ), the slave 12 b transmits a response signal as a reply packet (S 24 ), establishes a communication link with the master 12 a (S 25 ), and makes a transition to the communication state (S 26 ).

[0119] If the polling packet received at step S 22 is a response signal to a start signal transmitted at step S 29 described later (S 27 ), the slave 12 b establishes a communication link with the master 12 a (S 25 ) and makes a transition to the communication state (S 26 ).

[0120] If the polling packet received at step S 22 is any other than mentioned above such as null packet, etc., and the slave 12 b transmits a start signal (S 28 ), the slave 12 b transmits a start signal as a reply packet (S 29 ). A communication start request to be sent from the slave 12 b is set according to the high-level protocol in the sensor 13 or the slave 12 b and is held in the memory 33 , etc., until the process arrives at step S 28 .

[0121] The purpose of a step 30 is as follows: When the start signal is transmitted to the master 12 a , the slave 12 b remains in the communication link control state until reception of a response signal from the master 12 a (maximum remaining time=α×polling periods) and upon reception of the response signal, the route for the slave 12 b to make a transition to the communication state (steps S 27 , S 25 , S 26 ) is set.

[0122] If the slave 12 b does not transmit a start signal at step S 28 , whether or not the number of polling periods exceed the polling period counter value is determined (S 31 ). If the number of polling periods exceeds the value, the slave 12 b makes a transition from the communication link control state to the standby state (S 32 ); if the number of polling periods does not exceed the value, the polling period counter is decremented (S 33 ) and control returns to step S 22 .

[0123] FIG. 9 shows an operation flow of the slave 12 b in the communication state. This operation flow basically is executed by the CPU 32 functioning as the state control section for executing a communication packet transmission/reception program stored in the memory 33 . Steps 41 , 47 , and 48 in FIG. 9 are executed by the radio communication section 31 .

[0124] Upon reception of a polling packet (S 41 ), the slave 12 b determines whether or not the polling packet is directed to the slave 12 b . If the polling packet is directed to the slave 12 b , whether or not a communication packet from the master 12 a is superposed on the polling packet is determined (S 43 ). If a communication packet is superposed on the polling packet, the communication packet from the master 12 a is taken out and is transferred to the high-level protocol (S 44 ). Communication packet to be transmitted is read from the high-level protocol (S 45 ). If the communication packet to be transmitted exists (S 46 ), the communication packet is transmitted as a reply packet to the master 12 a (S 47 ); if the communication packet to be transmitted does not exist, a null packet is transmitted as a reply packet (S 48 ).

[0125] Communication termination indication from the high-level protocol is read (S 49 ). To terminate the communications (S 50 ), the slave 12 b makes a transition from the communication state to the communication link control state (S 51 ). To continue the communications, control returns to step S 41 and the operation flow is repeated.

[0126] (Operation Flow of Master)

[0127] FIG. 10 shows an operation flow of the master 12 a in the communication state. This operation flow corresponds to the scheme previously described with reference to FIG. 3 . It basically is executed by the CPU 22 functioning as a communication control section for executing a communication packet transmission/reception program stored in the memory 23 . Steps 63 , 67 , 75 and 71 in FIG. 10 are executed by the radio communication section 21 .

[0128] First, a polling packet to slave n (slave n is any of the slaves 1 to 3 in FIG. 3 ) is prepared (S 61 ) and whether or not a communication start request to be sent from the master 12 a to the slave n exists is determined (S 62 ). If a communication start request exists, a start signal as the polling packet is transmitted (S 63 ). The master 12 a waits for reception of a response signal from the slave n as a time limit state (S 64 ). Upon reception of a response signal, the master 12 a establishes a communication link with the slave n (S 65 ).

[0129] Whether or not a start signal was previously received from the slave n and a response signal needs to be sent to the slave n is determined by referencing response signal transmission request presence indication stored in the memory 23 (S 65 ′). If a response signal needs to be sent, the response signal is superposed on polling packet (S 66 ), the polling packet is sent to the slave n (S 67 ), and the master 12 a establishes a communication link with the slave n (S 68 ) If it is determined at step S 65 ′ that a response signal need not be sent to the slave n, whether or not a clearing signal needs to be transmitted to the slave n is determined (S 69 ). If a clearing signal needs to be transmitted, the clearing signal is superposed on polling packet (S 70 ), the polling packet is sent to the slave n (S 71 ) and the master 12 a opens the communication link with the slave n (S 72 ).

[0130] If it is determined at step S 69 that a clearing signal need not be transmitted to the slave n, whether or not a communication packet transmission request to the slave n exists is checked (S 73 ). A communication packet from the high-level protocol is superposed on polling packet (S 74 ) and the polling packet is transmitted (S 75 ).

[0131] After step S 68 or S 75 , the master 12 a waits for reception of a reply packet from the slave n as a time limit state (S 76 ) When a timeout occurs, the master 12 a polls the next slave n+1 (S 77 ) and determines whether or not polling of all slaves terminates in the polling period (S 84 ). If polling of all slaves does not terminate, control returns to step S 61 and the operation flow is repeated; if polling of all slaves terminates, the master 12 a enters a start wait state until the next polling period of the slave n. (S 85 ).

[0132] If a reply packet from the slave n is received at step S 76 , whether or not the reply packet is a clearing signal is determined (S 78 ), whether or not the reply packet is a communication packet is determined (S 80 ), and whether or not the reply packet is a start signal is determined (S 82 ). If the reply packet is a clearing signal, the communication link with the slave n is opened (S 79 ). If the reply packet is a communication packet, the communication packet is transferred to the high-level protocol (S 81 ). If the reply packet is a start signal, the response signal transmission request presence indication stored in the memory 23 , referenced at step S 65 is set to ON (presence) (S 83 ). Then, control goes to step S 77 .

[0133] FIG. 11 shows an operation flow of the master 12 a in the communication state corresponding to the scheme previously described with reference to FIG. 5 . The operation flow basically is also executed by the CPU 22 functioning as the communication control section for executing the communication packet transmission/reception program stored in the memory 23 . Steps 63 , 67 , 92 and 94 in FIG. 11 are executed by the radio communication section 21 .

[0134] In the operation flow, basically the same processing as in FIG. 10 is performed and steps S 91 to S 95 in FIG. 11 are added. Some of the steps previously described with reference to FIG. 10 are denoted-by the same numerals in FIG. 11 to show the relationship between steps S 91 to S 95 and the steps in FIG. 10 .

[0135] In the operation flow, steps S 91 to S 95 are executed in response to a request made by the high-level protocol. When a request is made by the high-level protocol, first the requested time is in the start wait state until the next polling period is determined (S 91 ). If the time is in the start wait state, a start signal is transmitted to the slave n (S 92 ) and the transmission of the start signal is repeated a predetermined number of times (S 93 ) and then control goes to step S 65 .

[0136] If the request time is not in the start wait state until the next polling period at step S 91 , a start signal to the slave n and a polling signal to be transmitted essentially at the time by the master 12 a are transmitted alternately (S 94 ) and the transmission is repeated a predetermined number of times (S 95 ) and then control goes to step S 65 .

[0137] The master 12 a always keeps track of the slaves 1 to 3 as to whether or not the slaves 1 to 3 are in the communication state. Thus, the master 12 a manages a management table stored in the memory 23 indicating whether or not each of the slaves 1 to 3 is in the communication state. The management table is a table updated dynamically; when a communication link with the slave n is established, the table entry corresponding to the slave n is set to ON (communication state) and when the communication link is opened, the table entry corresponding to the slave n is set to OFF (non-communication state).

[0138] As described above, the communication link is established or opened by a stimulus from the high-level protocol using the communication system 12 . That is, communication link setting or opening is started by a stimulus from the high-level protocol (request operation). Specifically, when the master 12 a starts the slave 12 b , the communication link is established when the master 12 a receives a response signal from the slave 12 b ; when the slave 12 b starts the master 12 a , the communication link is established when the master 12 a transmits a response signal to the slave 12 b . When the master 12 a transmits a clearing signal to the slave 12 b or when the master 12 a receives a clearing signal from the slave 12 b , the communication link is opened.

[0139] As described above, the communication system 12 is a fully wireless communication system which includes a local area radio network made up of the slaves and the master for enabling the slaves or the master to start communications and can be used continuously for many hours. Therefore, the communication system 12 can be preferably used with various monitoring systems requiring continuous monitoring for many hours.

[0140] As described above, in the communication system according to first aspect of the present invention, the slave can take any of the start state in which the slave can communicate with the master, the communication control state in which at least the slave can receive a start request signal transmitted by the master and makes a transition to the start state upon reception of the start request signal, or the standby state in which power is less consumed than in the start state or the communication control state. The slave further includes the state control section for repeatedly operating so that the slave in the standby state is switched to the communication control state at a predetermined timing and is switched to the standby state when the communication control state continues for a predetermined time period without receiving the start request signal.

[0141] In the described configuration, when communications between the slave and the master are not conducted, basically the slave can be placed in the standby mode for decreasing power consumption. The slave, which is in the standby state, is switched to the communication control state, for example, in a predetermined period under the control of the state control section and can monitor a start request sent from the master. Thus, if it becomes necessary for the master to communicate with the slave, the master can also transmit a start request signal at an independent timing of the slave for causing the slave to switch to the start state for conducting communications therebetween.

[0142] Therefore, according to the described configuration, the communication system that can start the slave in response to a start request made by the master while decreasing power consumption in the slave can be provided.

[0143] As the communication system according to the second aspect of the present invention, in the communication system of the first aspect of the present invention, communications between the master and the slave may be radio communications, the first communication section may transmit the start request signal by radio communications, and the second communication section may receive the start request signal by radio communications.

[0144] In the described configuration, a communication system wherein a fully wireless slave to which neither a communication line nor a power supply line is connected can be operated can be provided.

[0145] As the communication system according to the third aspect of the present invention, the communication system of the first or second aspect of the present invention may include a plurality of the slaves, wherein the master may switch assignment time periods of communications with the slaves in order, thereby communicating with the slaves, and the master transmits the start request signal, the master may transmit the start request signal in the assignment time period of communications with the slave to which the start request signal is transmitted.

[0146] In the described configuration, the master switches the assignment time periods of communications with the slaves in order, thereby communicating with the slaves as in polling processing started by the master, for example. The master can transmit the start request signal in the assignment time period of communications with the target slave as a part of polling processing, for example.

[0147] As the communication system according to the fourth aspect of the present invention, the communication system of the first or second aspect of the present invention may include a plurality of the slaves, wherein the master may switch assignment time periods of communications with the slaves in order, thereby communicating with the slaves, and the master transmits the start request signal, the master may transmit the start request signal using a time period between the assignment time periods of communications with the slaves.

[0148] In the described configuration, the master transmits the start request signal using the time period between the assignment time periods of communications with the slaves. Accordingly, if the number of slaves grows, enlarging of the start request signal transmission timing interval can be suppressed.

[0149] Consequently, while the time period of the communication control state in the slave is set to a shorter time period, the slave can receive the start request signal promptly and can respond to a start request from the mater promptly.

[0150] As the communication system according to the fifth aspect of the present invention, in the communication system of the fourth aspect of the present invention, the start request signal may be a signal that can cause two or more slaves to make a transition from the communication control state to the start state.

[0151] In the described configuration, when the master causes the two or more slaves to make a transition to the start state, it can cause the two or more slaves to make a transition to the start state by a common start request signal.

[0152] As the communication system according to the sixth aspect of