Abstract:
An electronic control unit charges capacitors of sensors to produce idle phase waveforms having different waveform parameters assigned to the sensors, and transmits nothing between the signal phase time periods. Each sensor detects the idle phase waveform parameter of the capacitor. The sensor responds to the ECU during the signal phase time period, if the detected parameter corresponds to predetermined waveform parameter.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on and incorporates herein by reference Japanese Patent Application No. 2008-290342 filed on Nov. 12, 2008. 
     FIELD OF THE INVENTION 
     The present invention relates to a communications network of a passenger protection system, which has an electronic control unit (ECU) and a plurality of sensors. The ECU is equipped in a passenger protection device such as an airbag for protecting passengers at time of collision of vehicles. The sensors are connected to the ECU and detect travel speed of a vehicle or collision. 
     BACKGROUND OF THE INVENTION 
     Various passenger protection devices such as an airbag and a seatbelt pretensioner are equipped in vehicles recently. The passenger protection system including such a passenger protection device includes, as shown in  FIG. 1 , front sensors  11   a ,  11   b  mounted at both front left and right sides of a vehicle  10 , safety sensors  13   a ,  13   b  mounted at a front or rear passenger seats in a passenger compartment, and a plurality of sensors (first to third sensors)  15   a  to  15   c ,  16   a  to  16   c  mounted at both left and right sides of the vehicle  10 . These sensors are connected to an electronic control unit (ECU)  18 , thus forming a communications network. Each of the sensors  11   a ,  11   b ,  13   a  to  13   c ,  15   a  to  15   c ,  16   a  to  16   c  detects travel speed or collision of the vehicle, and the ECU  18  activates airbags (not shown) based on the detected travel speed or collision. 
     In this communications network, the sensors  15   a  to  15   c ,  16   a  to  16   c  have respective switches in the inside parts and connected to the ECU  18  through buses. These switches are closed sequentially by initialization of setting addresses from the sensor closest to the ECU  18 , when electric power is supplied in the vehicle  10 . Specifically, the switch of the first sensor  15   a , which is closest to the ECU  18 , is set with its address and closed to connect the second sensor  15   b  to the ECU  18 . After setting an address to the sensor  15   b  by the ECU  18 , the switch of the sensor  15   b  is closed to connect the sensor  15   c  as the third sensor to the ECU  18 . The initialization is performed in this order. 
     In the communications between the ECU  18  and each sensor  15   a  to  15   c ,  16   a  to  16   c , voltage communications is performed from the ECU  18  to each sensor  15   a  to  15   c ,  16   a  to  16   c  and current communications is performed from each sensor  15   a  to  15   c ,  16   a  to  16   c  to the ECU  18 . 
     In the voltage communications, for example, “0” and “1” are used. “0” is an amplitude signal, if one-third (⅓) is 0 volt (V) and two-thirds (⅔) is 5 V with respect to the duty. “1” is an amplitude signal, if one-third (⅓) is 5 V and two-thirds (⅔) is 0 V with respect to the duty. Here, the duty is the ratio of time of 5V relative to one cycle time of the signal. 
     In the current communications, for example, “0” and “1” are used as well. However, “0” is a current signal of 0 milliamperes (mA) and “1” is a current signal of 10 mA. 
     It is assumed that the sensors  15   a  to  15   c  and the ECU  18  are bus-connected to each other through a power-side line  21  and a ground-side line  22  in series or in sequence as shown in  FIG. 2 . The sensors  15   a  to  15   c  have respective capacitors  24   a  to  24   c  at sides of the ECU  18  (input sides), and switches  26   a  to  26   c  to connect to the sensor of the following stage. 
     The voltage communications from the ECU  18  to each sensor  15   a  to  15   c  is indicated by (a) and the current communications from each sensor  15   a  to  15   c  to the ECU  18  is indicated by (b) in  FIG. 3 . 
     It is first assumed that the switches  26   a  to  26   c  of the sensors  15   a  to  15   c  are all in the off-state. It is also assumed that the capacitor  24   a  of the first sensor  15   a  is charged in response to a first charge command of the ECU  18  in the idle phase before time t 1 . In the following signal phase from time t 1  to time t 2 , the ECU  18  transmits to the first sensor  15   a  a first command to set a first address by the voltage communications. The first sensor  15   a  closes its switch  26   a  after receiving the first command and setting the first address, so that the second sensor  15   b  is connected to the ECU  18  therethrough. 
     The ECU  18  transmits a second charge command in the following idle phase from time t 2  to time t 3 . The capacitor  24   b  of the second sensor  15   b  is charged in response to the second charge command. In the signal phase from time t 3  to time t 4 , the first sensor  15   a  transmits to the ECU  18  a first response, which indicates completion of setting of the first address. The ECU  18  transmits a second command of second address setting to the second sensor  15   b  by the voltage communications. The second sensor  15   b  closes its switch  26   b  after receiving the second command and setting the second address, so that the third sensor  15   c  is further connected to the ECU  18  therethrough. After receiving the first response from the first sensor  15   a , the ECU  18  performs communications with the first sensor  15   a  by using the first address, which is included in the first response from the first sensor  15   a.    
     When the ECU  18  transmits a third charge command in the following idle phase from time t 4  to t 5 , the capacitor  24   c  of the third sensor  15   c  is charged. In the signal phase from time t 5  to time t 6 , the second sensor  15   b  transmits to the ECU  18  a second response, which indicates completion of setting the second address. The ECU  18  transmits a command of third address setting to the third sensor  15   c  by the voltage communications. The third sensor  15   b  sets its address after receiving the third command. After receiving the second response from the second sensor  15   b , the ECU  18  performs communications with the second sensor  15   b  by using the second address, which is included in the second response from the second sensor  15   b.    
     In the similar manner, the capacitor  24   c  of the third sensor  15   c  is charged in the idle phase from time t 6  to time t 7 . Then the third sensor  15   c  transmits to the ECU  18  a third response, which indicates completion of setting the third address, in the next signal phase from time t 7  to time t 8 . 
     JP 2007-215102A (U.S. Pat. No. 7,539,804) also discloses a conventional communications network, in which an ECU communicates with sensors by setting respective addresses in the similar manner as described above. According to the conventional communications networks, the ECU communicates with the sensors at communications speed of 150 to 200 kbps in the signal phase. In this instance, higher harmonics (noises) of frequencies corresponding to several times of the communications speed are generated, thus adversely affecting AM (amplitude modulation) radio frequency band of 510 kHz to 1710 kHz. Increased cost is necessitated to reduce such noises. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a communications network of a passenger protection system, which can perform communications between an electronic control unit and a plurality of sensors without adversely affecting an AM radio band nor requiring noise-countering, additional costs. 
     According to the present invention, a communications network of a passenger protection system having a passenger protection device, which protects passengers at time of vehicle collision, includes a plurality of sensors and an electronic control unit. The sensors include respective capacitors, which are electrically chargeable. The electronic control unit is configured to control the passenger protection device and is bus-connected to the sensors to control charging of the capacitors so that the sensors respond by electric charge of the capacitors. The electronic control unit is configured to control initialization of the sensors by controlling a state, in which an idle phase time period for charging the capacitors and a signal phase time period for receiving responses of the sensors are alternately repeated. The electronic control unit includes a first memory circuit for storing a plurality of waveform generation data provided in correspondence to the sensors differently one another so that the capacitors produce idle phase waveforms corresponding to the stored waveform generation data, and a communications control circuit for controlling respective charging of the sensors in the idle phase time period in accordance with the stored waveform generation data. Each of the sensors includes a second memory circuit for storing a parameter data corresponding to the stored waveform generation data provided thereto, a detection circuit for detecting a parameter data of the idle phase waveform of the capacitor, and a response circuit for responding to the electronic control unit in the signal phase time period when the detected parameter data of the idle phase waveform corresponds to the stored parameter data of the idle phase waveform. 
     Both the waveform generation data and the parameter data include either one of a charge time period, an inter-peak level and a peak number of each idle phase waveform produced by the capacitor in the idle phase time period. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
         FIG. 1  is a schematic diagram showing an ECU and a plurality of sensors of a communications network of a conventional passenger protection system in a vehicle; 
         FIG. 2  is a schematic diagram showing the ECU and bus-connected sensors of the communications network of the conventional passenger protection system in a vehicle; 
         FIG. 3  is a timing diagram showing signal waveforms at time of communications between the ECU and each sensor of the communications network in the conventional passenger protection system; 
         FIG. 4  is a block diagram showing a communications network of a passenger protection system according to a first embodiment of the present invention; 
         FIG. 5  is a timing diagram showing signal waveforms at time of communications between an ECU and a plurality of sensors of the communications network of the passenger protection system according to the first embodiment; 
         FIG. 6  is a waveform diagram showing detection of an idle phase time period by a time period detection circuit of the sensor in the communications network of the passenger protection system according to the first embodiment; 
         FIG. 7  is a block diagram showing a communications network of a passenger protection system according to a second embodiment of the present invention; 
         FIGS. 8A to 8C  are waveform diagrams showing idle phase waveforms having respective inter-peak waveform levels in the communications network of the passenger protection system according to the second embodiment; 
         FIG. 9  is a timing diagram showing signal waveforms at time of communications between an ECU and a plurality of sensors of the communications network of the passenger protection system according to the second embodiment; 
         FIG. 10  is a block diagram showing a communications network of a passenger protection system according to a third embodiment of the present invention; 
         FIGS. 11A to 11C  are waveform diagrams showing idle phase waveforms having respective number of peaks in the communications network of the passenger protection system according to the third embodiment; 
         FIG. 12  is a timing diagram showing signal waveforms at time of communications between an ECU and a plurality of sensors of the communications network of the passenger protection system according to the third embodiment; 
         FIG. 13  is a waveform diagram showing a synchronization signal transmitted from an ECU to each sensor in a communications network of a passenger protection system according to a modified embodiment of the present invention; and 
         FIG. 14  is a timing diagram showing commands transmitted from an ECU to a plurality of sensors and responses to the commands from the sensors according to a modified embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will be described in detail with reference to various embodiments, in which the same or similar parts are designated by the same or, similar parts and the same or similar description thereof is omitted for brevity. 
     First Embodiment 
     Referring to  FIG. 4 , a communications network of a passenger protection system is configured generally in the similar manner as the conventional communications network shown in  FIG. 1 . In  FIG. 4 , however, only a plurality of sensors  15   a  to  15   c , which are mounted as series-connected first to third sensors at the right side of a vehicle  10 , are shown as being bus-connected to an ECU  18 . 
     The ECU  18  includes a communications control circuit  31  and a first memory circuit  32 . The first sensor  15   a  includes a second memory circuit  35 , a time period detection circuit  36 , a switch control circuit  37  and a response circuit  38 . The second sensor  15   b  and the third sensor  15   c  are also configured in the same manner as the first sensor  15   a.    
     The memory circuits  32  of the sensors  15   a  to  15   c  store therein respective idle phase time periods Ta, Tb, Tc, which are set to be different one another. The idle phase time periods Ta, Tb, Tc are also referred to as a first sensor time period, a second sensor time period, a third sensor time period, respectively, as waveform generation data of each sensor  15   a  to  15   c.    
     The communications control circuit  31  of the ECU  18  is configured to control communications with the sensors  15   a  to  15   c  by charging capacitors  24   a  to  24   c  of the sensors  15   a  to  15   c  during respective idle phase time periods Ta to Tc as indicated by (a) in  FIG. 5 . The communications control circuit  31  however transmits nothing during each signal phase between two of the idle phase time periods Ta to Tc. The capacitors  24   a  to  24   c  are charged to supply electric power for communications of the sensors  15   a  to  15   c.    
     The memory circuit  32  of the ECU  18  stores therein the idle phase time periods Ta to Tc of the sensors  15   a  to  15   c . The idle phase time periods Ta to Tc are parameter data of the charge voltage waveform of the capacitors  24   a  to  24   c  and correspond to the waveform generation data stored in the first memory circuit  32 . 
     The time period detection circuit  36  of the first sensor  15   a  detects, as a parameter detection circuit, the idle phase time period from the charge period of the capacitor  24   a  and checks whether the detected time period corresponds to the idle phase time period Ta pre-stored in the memory circuit  35 . The detection circuit  36  outputs a check result to the switch control circuit  37  and the response circuit  38 . 
     In detecting the idle phase time period, as shown in  FIG. 6 , the time period detection circuit  36  starts measuring the time period at time the charge voltage of the capacitor  24   a  developed by charging the capacitor  24   a  rises from 0 V to a predetermined threshold level, for example 4 V. The detection circuit  36  stops measuring the time period at time the charge voltage falls below a predetermined threshold level, for example 20 V, after reaching an upper limit level 25 V, respectively. The time period from starting to stopping the measurement is detected as the idle phase time period. 
     The switch control circuit  37  closes the switch  26   a  thereby to connect the second sensor  15   b  to the ECU  18 , when the check result of the detection circuit  36  indicates that the detected time period corresponds to the idle phase time period Ta. When the check result of the detection circuit  36  indicates that the detected time period does not correspond to the idle phase time period Ta, the switch control circuit  37  does not close the switch  26   a.    
     When the check result of the detection circuit  36  indicates that the detected time period corresponds to the idle phase time period Ta, the response circuit  38  transmits a first response to the ECU  18  as shown in  FIG. 5 . This first response is transmitted in the signal phase period (time t 4  to time t 5 ), which follows the idle phase time period Tb subsequent to the check operation (time t 2  to time t 3 ). The first response indicates that the idle phase time period Ta for the first sensor  15   a  has been received. 
     The ECU  18  thus recognizes the completion of initialization of the first sensor  15   a  by receiving the first response. The initialization of the first sensor  15   a  may be a turn-on of the first switch  26   a  in the first sensor  15   a.    
     The initialization of the sensors  15   a  to  15   c  is performed in the communications network of the passenger protection system in the following manner, when electric power is supplied in the vehicle  10 . 
     When the power supply is started in the vehicle  10 , the communications control circuit  31  of the ECU  18  controls the first sensor  15   a  to charge the capacitor  24   a  during the idle phase time period Ta stored in the memory circuit  32  as a first sensor time period Ta. That is, the capacitor  24   a  is charged during the first sensor time period Ta from time t 1  to time t 2  as shown in  FIG. 5 . 
     In the first sensor  15   a , in the idle phase time period (time t 2  to t 3 ), the time period detection circuit  36  measures the time period, in which the capacitor  24   a  is charged, to detect the idle phase time period. The detection circuit  36  further checks whether the detected time period substantially equals the idle phase time period Ta stored in the memory circuit  35 . If the check result indicates that both time periods correspond to each other, this check result is applied to the switch control circuit  37  and the response circuit  38 . 
     The switch control circuit  37  responsively closes the switch  26   a  thereby to connect the second sensor  15   b  to the ECU  18 . The response circuit  38  transmits to the ECU  18  the first response, which indicates that the idle phase time period Ta has been received. This first response is transmitted in the signal phase time period, that is, from time t 4  to time t 5 , which follows the idle phase time period Tb provided for the second sensor  15   b  subsequent to the preceding idle phase time period Ta for the first sensor  15   a . The ECU  18  does not transmit any signals in this signal phase time period. The communications control circuit  31  of the ECU  18  thus recognizes the completion of initialization of the sensor  15   a  upon receiving the first response. 
     In the similar manner, in the second sensor  15   b , the idle phase time period Tb, which is from time t 3  to time t 4 , is detected and the switch  26   b  is closed to connect the third sensor  15   c  to the ECU  18 . In the signal phase time period Tc from time t 6  to time t 7 , the second response is transmitted from the second sensor to the ECU  18 . Further, in the third sensor  15   b , the idle phase time period Tc, which is from time t 5  to time t 6 , is detected and the switch  26   c  is closed. In the signal phase time period, from time t 8  to time t 9 , the third response is transmitted from the third sensor to the ECU  18 . The communications control circuit  31  of the ECU  18  recognizes the completion of initialization of the sensors  15   b  and  15   c  upon receiving the second and third responses. 
     The sensors  15   a  to  15   c  may be connected in sequence without the switches  26   a  to  26   c  and the switch control circuits  37 . 
     According to the first embodiment described above, the sensors  15   a  to  15   c  are provided with respective capacitor charge time periods as the idle phase time periods Ta to Tc, which are different from one another. The ECU  18  performs the capacitor charge control for the capacitors  26   a  to  26   c  during the idle phase time periods Ta to Tc and does not perform any signal transmission during the signal phase time periods. 
     In the sensors  15   a  to  15   c , the idle phase time periods T are detected by measuring the charge time periods of the capacitors  26   a  to  26   c . If the detected idle phase time T periods correspond to the stored idle phase time periods Ta to Tc, the response signals are transmitted to the ECU  18  during the signal phase time periods. The ECU  18  thus recognizes each sensor by receiving the corresponding response. 
     The ECU  18  thus does not transmit any signals in the signal phases in controlling the initialization of the sensors  15   a  to  15   c , and hence the ECU  18  can specify each sensor  15   a  to  15   c  by receiving the response in the signal phases. 
     As a result, it is restricted that higher harmonics (noises) of frequencies corresponding to several times of the communications speed are generated, thus adversely affecting the AM radio frequency band. Further, no increased cost is necessitated to counter such noises. That is, the communications from the ECU  18  to the sensors  15   a  to  15   c  does not adversely affect the AM radio frequency band nor increase costs. 
     Second Embodiment 
     According to a second embodiment, as shown in  FIG. 7 , each sensor  15   a  to  15   c  includes a level detection circuit  46  as a parameter detection circuit in place of the time period detection circuit  36  of the first embodiment. Further, the memory circuit  32  of the ECU  18  stores predetermined idle phase waveform generation data LaD to LcD in place of the time periods Ta to Tc of the first embodiment. 
     The idle phase waveform generation data LaD, LbD, LcD are provided to generate idle phase waveforms LaT, LbT, LcT for the sensors  15   a ,  15   b ,  15   c  as shown in  FIGS. 8A ,  8 B,  8 C, respectively. The idle phase waveforms LaT to LcT have inter-peak voltage levels La to Lc, which are different one another. The levels La, Lb, Lc are provided as parameter data and present between two peaks A 1  and A 2 , between two peaks B 1  and B 2 , between two peaks C 1  and C 2 . 
     The idle phase waveforms LaT to LcT are charge voltage waveforms of the capacitors  24   a  to  24   c  of the sensors  15   a  to  15   c , respectively. Each capacitor  24   a  to  24   c  is effectively charged during the time period T in the second peak waveform A 2  to C 2  in each waveform cycle. Each idle phase waveforms LaT to LcT is shown in  FIG. 9  in a simplified waveform having only one peak. 
     The communications control circuit  31  is configured to charge the capacitors  24   a  to  24   c  in accordance with the idle phase waveform generation data LaD to LcD stored in the memory circuit  32  so that the capacitors  24   a  to  24   c  generate the idle phase waveforms LaT to La, respectively, in the time sequence as shown in  FIG. 9 . The communications control circuit  31  transmits no signals during the signal phase time period as in the first embodiment. 
     The memory circuits  35  of the sensors  15   a  to  15   c  store predetermined inter-peak levels La to Lc as parameter data of the waveforms and different from one another, respectively, in place of the time periods Ta to Tc of the first embodiment. 
     The level detection circuit  46  is configured as the parameter detection circuit to detect the inter-peak level La from the idle phase waveform LaT, which is developed when the capacitor  24   a  is charged. The level detection circuit  46  further checks whether the detected inter-peak level La corresponds to its level stored in the memory circuit  35 , and outputs its check result to the switch control circuit  37  and the response circuit  38 . 
     The switch control circuit  37  closes the switch  26   a  of the first sensor  15   a  to connect the second sensor  15   b  to the ECU  18 , when the check result of the waveform level detection circuit  46  indicates that the detected level corresponds to the stored level La. The switch control circuit  37  does not close the switch  26   a , when the check result indicates no correspondence. 
     When the check result of the detection circuit  36  indicates that the detected level corresponds to the stored level La, the response circuit  38  transmits a first response to the ECU  18  as shown in  FIG. 9 . This first response is transmitted in the signal phase period (time t 4  to time t 5 ), which follows the idle phase time period subsequent to the check operation (time t 2  to time t 3 ). The first response indicates that the waveform level. La for the first sensor  15   a  has been received. 
     The communications control circuit  31  of the ECU  18  thus recognizes the completion of initialization of the first sensor  15   a  by receiving the first response. 
     The initialization of the sensors  15   a  to  15   c  is performed in the communications network of the passenger protection system in the following manner, when electric power is supplied in the vehicle  10 . 
     When the power supply is started in the vehicle  10 , the communications control circuit  31  of the ECU  18  controls the first sensor  15   a  to charge the capacitor  24   a  by the idle phase waveform generation data LaD stored in the memory circuit  32 . That is, the capacitor  24   a  is charged during the idle phase time period from time t 1  to time t 2  shown in  FIG. 9  in the waveform of  FIG. 8A . 
     In the first sensor  15   a , in the signal phase period (from time t 2  to time t 3 ), the waveform level detection circuit  36  detects the inter-peak waveform level La from the idle phase waveform LaT produced when the capacitor  24   a  is charged. The detection circuit  36  further checks whether the detected level is the same as the inter-peak waveform level La stored in the memory circuit  35 . If the check result indicates that both levels correspond to each other, this check result is applied to the switch control circuit  37  and the response circuit  38 . 
     The switch control circuit  37  responsively closes the switch  26   a  thereby to connect the second sensor  15   b  to the ECU  18 . The response circuit  38  transmits to the ECU  18  the first response, which indicates that the inter-peak waveform level La for the first sensor  15   a  has been received. This first response is transmitted in the signal phase time period, that is, from time t 4  to time t 5 , which follows the idle phase time period provided for the second sensor  15   b  subsequent to the preceding idle phase period for the first sensor  15   a . The communications control circuit  31  of the ECU  18  recognizes the completion of initialization of the sensor  15   a  upon receiving the first response. 
     In the similar manner, in the second sensor  15   b , the waveform level Lb produced in the idle phase time period from time t 3  to time t 4  is detected in the time period from time t 3  to time t 4  and the switch  26   b  is closed to connect the third sensor  15   c  to the ECU  18 . In the signal phase time period from time t 6  to time t 7 , the second response is transmitted from the second sensor  15   b  to the ECU  18 . Further, in the third sensor  15   b , the waveform level Lc is detected in the time period from time t 5  to time t 6 , and the switch  26   c  is closed. In the signal phase time period from time t 8  to time t 9 , the third response is transmitted from the third sensor  15   c  to the ECU  18 . The communications control circuit  31  of the ECU  18  recognizes the completion of initialization of the sensors  15   b  and  15   c  upon receiving the second and third responses, respectively. 
     The sensors  15   a  to  15   c  may be connected in series without the switches  26   a  to  26   c  and the switch control circuits  37 . 
     According to the second embodiment described above, the sensors  15   a  to  15   c  are provided with respective inter-peak waveform levels La to Lc, which are different from one another. The ECU  18  performs the capacitor charge control for the capacitors  26   a  to  26   c  in accordance with the idle phase waveform generation data LaD to LcD, and does not perform any signal transmission during the signal phase time period. 
     In the sensors  15   a  to  15   c , the inter-peak waveform levels La to Lc are detected from the idle phase waveforms LaT to LcT produced by charging the capacitors  26   a  to  26   c . If the detected waveform levels correspond to the stored respective levels, the response signals are transmitted to the ECU  18  during the signal phase time periods. The ECU  18  recognizes each sensor by receiving the corresponding response. 
     The ECU  18  thus does not transmit any signals in the signal phases in controlling the initialization of the sensors  15   a  to  15   c , and hence the ECU  18  can specify each sensor  15   a  to  15   c  by receiving the responses in the signal phases. 
     As a result, it is restricted that higher harmonics (noises) of frequencies corresponding to several times of the communications speed are generated, thus adversely affecting the AM radio frequency band. Further, no increased cost is necessitated to counter such noises. That is, the communications from the ECU to the sensors does not adversely affect the AM radio frequency band nor increase costs. 
     Third Embodiment 
     According to a third embodiment, as shown in  FIG. 10 , each sensor  15   a  to  15   c  includes a peak number detection circuit  56  as a parameter data detection circuit in place of the time period detection circuit  36  of the first embodiment and the inter-peak waveform level detection circuit  46  of the second embodiment. Further, the memory circuit  32  of the ECU  18  stores predetermined idle phase waveform generation data PaD to PcD in place of the time periods Ta to Tc of the first embodiment and the idle phase waveform generation data LaD to LcD of the second embodiment. 
     The idle phase waveform generation data PaD, PbD, PcD are provided to generate idle phase waveforms PaT, PbT, PcT for the sensors  15   a ,  15   b ,  15   c  as shown in  FIGS. 11A ,  11 B,  11 C, respectively. The idle phase waveforms PaT to PcT have different number of peaks (different peak numbers). The waveform PaT for the first sensor  15   a  has two peaks A 1 , A 2 , the waveform PbT for the second sensor  15   b  has three peaks B 1 , B 2 , B 3 , and the waveform PcT for the third sensor  15   c  has four peaks C 1 , C 2 , C 3 , C 4 . 
     The idle phase waveforms PaT to PcT are charge voltage waveforms of the capacitors  24   a  to  24   c  of the sensors  15   a  to  15   c , respectively. Each capacitor  24   a  to  24   c  is effectively charged during the time period T in the last peak waveform A 2  to C 4  in each waveform cycle. Each idle phase waveforms PaT to PcT is shown in  FIG. 12  in a simplified waveform having only one peak. 
     The communications control circuit  31  is configured to charge the capacitors  24   a  to  24   c  in accordance with the idle phase waveform generation data PaD to PcD stored in the memory circuit  32  so that the capacitors  24   a  to  24   c  generate the idle phase waveforms PaT to PcT, respectively, in the time sequence as shown in  FIG. 12 . The communications control circuit  31  transmits no signals during the signal phase time period between the two idle phase time periods, in each of which the idle phase waveform PaT to PcT is generated. 
     The memory circuits  35  of the sensors  15   a  to  15   c  store the predetermined number Pa (=2), Pb (=3), Pc (=4) of peaks (peak number) as parameter data and are different from one another, respectively. 
     The peak number detection circuit  56  is configured to detect, as the parameter detection circuit, the number of peaks peak Pa from the idle phase waveform PaT, which is developed when the capacitor  24   a  is charged. The peak number detection circuit  56  further checks whether the detected peak number corresponds to its stored peak number Pa, and outputs its check result to the switch control circuit  37  and the response circuit  38 . 
     The switch control circuit  37  closes the switch  26   a  of the first sensor  15   a  to connect the second sensor  15   b  to the ECU  18 , when the check result of the peak number detection circuit  56  indicates that the detected peak number corresponds to the stored peak number Pa. The switch control circuit  37  does not close the switch  26   a , when the check result indicates no correspondence. 
     When the check result of the peak number detection circuit  56  indicates that the detected peak number corresponds to the stored peak number Pa, the response circuit  38  transmits a first response to the ECU  18  as shown in  FIG. 12 . This first response is transmitted in the signal phase period (time t 4  to time t 5 ), which follows the idle phase time period (time t 3  to time t 4 ) subsequent to the check operation (time t 2  to time t 3 ). The first response indicates that the peak number Pa for the first sensor  15   a  has been received. 
     The communications control circuit  31  of the ECU  18  thus recognizes the completion of initialization of the first sensor  15   a  by receiving the first response. 
     The initialization of the sensors  15   a  to  15   c  is performed in the communications network of the passenger protection system in the following manner, when electric power is supplied in the vehicle  10 . 
     When the power supply is started in the vehicle  10 , the communications control circuit  31  of the ECU  18  controls the first sensor  15   a  to charge the capacitor  24   a  by the idle phase waveform generation data PaD stored in the memory circuit  32 . That is, the capacitor  24   a  is charged during the idle phase time period from time t 1  to time t 2  shown in  FIG. 12  in the waveform having two peaks A 1 , A 2  as  FIG. 11A . 
     In the first sensor  15   a , in the signal phase time period (time t 2  to time t 3 ), the peak number detection circuit  36  detects the peak number from the idle phase waveform PaT produced when the capacitor  24   a  is charged. The peak number detection circuit  36  further checks whether the detected peak number corresponds to the predetermined peak number Pa stored in the memory circuit  35 . If the check result indicates that both peak numbers correspond to each other, this check result is applied to the switch control circuit  37  and the response circuit  38 . 
     The switch control circuit  37  responsively closes the switch  26   a  thereby to connect the second sensor  15   b  to the ECU  18 . The response circuit  38  transmits to the ECU  18  the first response, which indicates that the peak number Pa for the first sensor  15   a  has been received. This first response is transmitted in the signal phase time period, that is, from time t 4  to time t 5 , which follows the idle phase time period (time t 3  to time t 4 ) and is subsequent to the preceding signal phase period (time t 2  to time t 3 ). The communications control circuit  31  of the ECU  18  recognizes the completion of initialization of the sensor  15   a  upon receiving the first response. 
     In the similar manner, in the second sensor  15   b , the peak number Pb produced in the idle phase time period from time t 3  to time t 4  is detected and the switch  26   b  is closed to connect the third sensor  15   c  to the ECU  18 . In the signal phase time period from time t 6  to time t 7 , the second response is transmitted from the second sensor  15   b  to the ECU  18 . Further, in the third sensor  15   b , the peak number Pc is detected, and the switch  26   c  is closed. The third sensor  15   c  is connected to the ECU  18  and the capacitor  24   c  is charged in accordance with the idle phase waveform generation data PcD stored in the memory circuit  32  of the ECU  18 . In the signal phase time period from time t 8  to time t 9 , the third response is transmitted from the third sensor  15   c  to the ECU  18  if the peak number detection circuit  56  detects that the detected peak number corresponds to the peak number Pc stored in the sensor  15   c . The communications control circuit  31  of the ECU  18  recognizes the completion of initialization of the sensors  15   b  and  15   c  upon receiving the second and third responses, respectively. 
     The sensors  15   a  to  15   c  may be connected in series without the switches  26   a  to  26   c  and the switch control circuits  37 . 
     According to the third embodiment described above, the sensors  15   a  to  15   c  are provided with respective peak numbers Pa to Pc as the parameter data, which are different from one another. The ECU  18  performs the capacitor charge control for the capacitors  26   a  to  26   c  in accordance with the idle phase waveform generation data PaD to PcD, and does not perform any signal transmission during the signal phase time period. 
     In the sensors  15   a  to  15   c , the peak numbers are detected from the idle phase waveforms PaT to PcT produced by charging the capacitors  26   a  to  26   c . If the detected peak numbers correspond to the stored respective peak numbers, the response signals are transmitted to the ECU  18  during the signal phase time periods. The ECU  18  recognizes each sensor  15   a  to  15   c  by receiving the corresponding response. 
     The ECU  18  thus does not transmit any signals in the signal phases in controlling the initialization of the sensors  15   a  to  15   c , and hence the ECU  18  can specify each sensor  15   a  to  15   c  by receiving the responses in the signal phases. 
     As a result, it is restricted that higher harmonics (noises) of frequencies corresponding to several times of the communications speed are generated, thus adversely affecting the AM radio frequency band. Further, no increased cost is necessitated to counter such noises. That is, the communications from the ECU  18  to the sensors  15   a  to  15   c  does not adversely affect the AM radio frequency band nor increase costs. 
     Other Embodiments 
     In the foregoing embodiments, the ECU  18 , specifically the communications control circuit  31 , may be configured to generate a pulse-shaped synchronization signal  61  in the signal phase time period as shown in  FIG. 13 , and each sensor  15   a  to  15   c  may be configured to include a synchronization control circuit, which detects the synchronization signal  61  and synchronize its operation to the synchronization signal  61 . 
     This configuration ensures synchronization of communications between the ECU  18  and the sensors  15   a  to  15   c.    
     In the foregoing embodiments, the ECU  18 , specifically the communications control circuit  31 , may be configured to transmit a predetermined command in one of the signal phase time periods, for example, from time t 4  to time t 5 , as shown in (a) of  FIG. 14 . The predetermined command may be for resetting each sensor  15   a  to  15   c , diagnosing each sensor  15   a  to  15   c , and the like. Each sensor  15   a  to  15   c , specifically the response circuit  38 ; may be configured to detect the command and performs operation corresponding to the command with priority in the next signal phase time period, that is, time t 6  to time t 7 . 
     This configuration ensures that each sensor  15   a  to  15   c  performs the operation of the transmitted command with priority.