Communicator and communication method and system
TECHNICAL FIELD 
     The present invention relates to communicating devices for use with a plurality of remote devices having communication capability and specifically for interconnecting with a communication bus for interrogating a plurality of emergency lighting and exit sign type devices as to their operating condition as well as to initiate and observe results of self-testing. The invention also relates to a communication method and system used as part of a communicator (which may be portable) which has the ability to determine a remote device&#39;s communication speed and to adapt thereto. 
     BACKGROUND OF THE INVENTION 
     The present invention is directed to a communication method and system so as to provide for communication between a host communicator unit and one or more remote units in a noisy communication environment. The host communicator may be portable and interconnects with a plurality of remote units via a communication bus. These remote units may be emergency lighting or exit sign devices with self-testing capability as well as communication capability. Background concerning such emergency lighting units and exit signs having self-testing capability is disclosed in co-pending U.S. Application, Ser. No. 07/048,530, filed May 8, 1987, for an Emergency Lighting Supervisory System, assigned to the present assignee, and hereby incorporated by reference. 
     One of the problems encountered in a system designed to be interconnected with a plurality of remote units using a bus, wherein a plurality of such units are connected across the bus, is the requirement that communications be reliable and that they operate in environments where such emergency lighting and exit signs are located, typically in commercial and industrial establishments which may have high ambient electrical noise due to electrical motors, fluorescent lighting fixtures and the like. 
     The present invention addresses this problem by incorporating a data detection method which provides for the automatic adjustment of the receiver associated with the communicator so as to determine the remote unit&#39;s actual transmission rate which may vary widely from a nominal transmission rate. It is desirable to reliably detect such remote units since such units are significantly less expensive to manufacture if crystal controlled transmitters are not utilized. 
     The present invention detects the actual transmission rate from the remote unit by use of a start of message (SOM) header element from the remote unit which thereby allows for the susequent accurate detection of data &#34;1&#34; bits and data &#34;0&#34; bits received from the remote unit. The SOM also denotes the start of the entire message from the transmitting unit. 
     The present invention then, through its associated electronics, allows the user at the host communicator to interrogate one or more of remote units as to their operating status as well as to initiate self-testing operations. 
     SUMMARY OF THE INVENTION 
     A communication method and system for error-free data detection is disclosed, directed particularly for use between a host communicator (which may be portable) and one or more remote units for receipt of information from those remote units in a noisy electrical environment. The communication method employs a data detection method so as to insure a high probability that the data received is error-free and furthermore to receive and properly detect such incoming data for remote units having transmission speeds which may vary widely from a nominal transmission speed. 
     This data detection method is used in conjunction with a host communicator that can interrogate the remote units concerning their status and to initiate self-tests at these remote units. 
     The data detection method incorporates three basic data elements, namely: a start of message header, a data &#34;1&#34; bit and a data &#34;0&#34; bit. These elements are defined in terms of the mark/space interval ratio (logic one and logic zero ratio) which in turn have durations expressed in units of time called &#34;ticks&#34;. The nominal tick time for incoming data from a remote unit can vary by approximately plus or minus thirty percent and the host communicator can accommodate and accurately detect such a variation in the nominal tick time so as to validly detect a start of message header, data &#34;1&#34; bit and a data &#34;0&#34; bit. 
     In operation, the first element received from a remote unit is a start of message header and this header is detected in a manner so as to detect the variations in actual tick time from the nominal tick time. This detection analysis is completed prior to the completion of the start of message header so that immediately following data from the remote unit can be accurately detected by the host unit. In this manner, valid data reception is insured over a wide range of remote unit data transmission rates. 
     Furthermore, the particular sampling techniques used to detect a data &#34;1&#34; bit and a data &#34;0&#34; bit further insures detection even when induced electrical &#34;noise&#34; is present on the communication bus. 
     OBJECTS OF THE INVENTION 
     It is therefore a principal object of the present invention to provide a communication method and system having a data detection technique which can accurately determine the incoming data transmission rate from a remote unit over a wide variation about a nominal transmission rate. 
     A further object of the present invention is to provide a communication method and system as described above wherein the incoming data comprises a start of message header which in turn is related to a plurality of time intervals (tick times) and wherein the variation of the actual tick time from the nominal tick time is determined by sensing a logic one to logic zero transition after receipt of a valid first portion of the start of message header. 
     A further object of the present invention is to provide a communication method and system as defined above wherein the determination of the actual tick time is used for determining incoming data &#34;1&#34; bits and data &#34;0&#34; bits. 
     A still further object of the present invention is to provide a communication method and system wherein all data elements are sampled a plurality of times during each data element&#39;s overall duration so as to insure error-free data detection. 
     A still further object of the present invention is a communicator (which may be portable) incorporating the communication method and system as defined above so as to interrogate one or more remote units interconnected to the communicator via a communication bus. 
     Another object of the present invention is a communication method and system that uses consecutive readings of the bus to determine the presence of a valid logic zero to logic one transition and a logic one to logic zero transition. 
     A still further object of the present invention is a communication method and system that requires at least a majority of a plurality of consecutive readings to be a logic zero or a logic one for a corresponding valid logic zero or logic one to be determined. 
     Another object of the present invention is a communication method and system that resets the timer associated with determining a logic &#34;0&#34; bit or a logic &#34;1&#34; bit based upon detecting a logic zero one transition. 
     Other objects of the present invention will in part be obvious and will in part appear hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the nature and objects of the present invention, reference should be made to the following detailed description taken in connection with the accompanying drawings, in which: 
     FIG. 1 is a system configuration diagram showing a host communicator and a plurality of remote units interconnected by a communication bus employing the communication method and system of the present invention. 
     FIG. 2, comprising FIGS. 2A-2G is an overall schematic diagram of the logic circuitry used in the host communicating device shown in FIG. 1. 
     FIG. 2H is a diagram showing how FIGS. 2A-2G are put together to form FIG. 2. 
     FIG. 3 is a schematic diagram of the line bias and power source devices shown in FIG. 1. 
     FIG. 4 is an electrical waveform showing a nominal start of message header (SOM). FIG. 4A is an electrical waveform of an actual start of message header as well as the sampling of this header by the host communicator so as to determine the variation between the actual tick time interval and the nominal tick time interval. 
     FIG. 5 is an electrical waveform for a data &#34;1&#34; bit showing the sampling intervals and the tick time intervals comprising this data element. 
     FIG. 6 is a waveform showing the data &#34;0&#34; bit showing the sampling intervals to detect this data element by the host communicator and the tick time intervals associated with this data element. 
     FIG. 7 shows the overall message which may be sent by a remote unit or the host communicator. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     As best seen in FIG. 1, an overall system 20 employing a data communication method and system according to the present invention incorporates a host communicator 22 (which may be portable), one or more remote units 24 which interconnect with the host communicator 22 by means of communication bus 26, and a line bias and power source 28. The communicator bus includes lines 26&#39; and 26&#34; which are maintained in a logic one state by the bias module unless communication information is present. As shown in FIG. 3, this module also can provide power to the host communicator. 
     In the preferred embodiment of the present invention, the host communicator 22 is a portable device having logic circuitry as set forth in FIG. 2 comprising FIGS. 2A through 2G. 
     A line bias and power source 28 connects to communication lines 26&#39; and 26&#34; so as to differentially bias these two lines to be in a plus 5 volt configuration line (26&#39; positive with respect to line 26&#34;), thereby representing a logic one state. Outputs 28&#39; and 28&#34; provide rectified low voltage power to communicator 22 for powering the communicator after rectification via a power supply such as that disclosed in applicant&#39;s co-pending application Ser. No. 07/145,225, filed Jan. 19, 1988, hereby incorporated by reference. 
     In operation, each remote unit has a unique address which when sensed by that remote unit, causes that remote unit to respond to commands from host communicator 22. The host communicator can thus determine the present status of the remote unit and can also initiate various test procedures to be conducted by the remote unit with subsequent transfer of the test results to the communicator. 
     Details concerning the differential receiver circuit used in the remote units are presented in U.S. Pat. No. 4,742,309, issued May 3, 1988,  whose disclosure is hereby incorporated by reference. 
     In operation, a remote unit when it senses its address in a transmitted message, responds with its own message which is received by communicator 22. The transmitted message by communicator 22 is produced by driver 41 with the incoming data received by receiver 42. The receivers 30 associated with each remote unit correspond to the receivers disclosed in corresponding application Ser. No. 06/948,313. 
     As seen in FIG. 7, each transmission message, whether generated by the communicator 22 or by a remote unit 24, comprises the same overall format; namely, a start of message header (SOM) eight address bits (A 0  through A 7 ), five status bits (S 0  through S 4 ), one parity bit, and two end of message trailer bits (EOM) which must be logic &#34;0&#34; bits. 
     As seen in FIG. 4, the start of message header has a specific format; namely, a logic zero to logic one transition, followed by 28 tick times in which the logic one state is maintained, followed by four tick times in which the logic zero state is maintained. The format for a data &#34;1&#34; bit comprises a logic one state for twelve tick times and a logic zero state for four tick times as shown in FIG. 5. The format for a data &#34;0&#34;  bit comprises a logic one state for four tick times, followed by a logic zero state for twelve tick times. 
     The nominal tick time is 625 microseconds. The communicator 22 as seen in FIG. 22A incorporates a crystal 50 so that the transmitted output on lines 52 and 52&#39; maintains a precise 625 microsecond tick time length. Thus the nominal start of message header comprises twenty-eight ticks or 17,500 micro-seconds (17.5 milliseconds) in a logic one state and 2,500 micro-seconds (2.5 ms) in a logic zero state. The logic one state is defined as a plus 5 volt differential signal between lines 26&#39; and 26&#34; (line 26&#34; positive with respect to line 26&#39;) while a logic zero state is defined as a minus 5 volt differential signal between lines 26&#39; and line 26&#34;. 
     The data &#34;1&#34; bit as shown in FIG. 5 comprises a logic one state for twelve tick times, that is for 7.5 ms and a logic zero state for four tick times, that is for 2.5 ms. 
     Conversely, the data &#34;0&#34; bit comprises a logic one for 2.5 ms corresponding to four tick times and a logic zero state for 7.5 ms, corresponding to twelve tick times. 
     Determination of the Actual Tick Time Associated With a Remote Unit 
     The remote unit&#39;s transmitter 38 is not crystal controlled nor tightly controlled by other means and therefore its nominal tick time can vary by as much as plus or minus thirty percent of the nominal 625 microsecond tick time duration. Because of this wide range in the actual tick time associated with any remote unit, it is necessary that the communicator 22 be able to determine the actual tick time of a transmitting remote unit in order to be able to correctly interpret the subsequent incoming data. Since as shown in FIG. 7 all transmission messages begin with a start of message header, the start of message header not only informs the communicator that a remote unit is transmitting a message, but also provides the necessary information to determine the actual tick time associated with that remote unit. 
     FIG. 4A is a waveform of a typical SOM header from a remote unit. Just prior to the start of message header, the remote unit transmits a short logic zero for at least 100 microseconds, but typically 1 ms. This is shown in FIG. 4A by reference 64 for the initial logic one to logic zero and logic zero to logic one transitions at the beginning of the SOM waveform. Once the communicator detects a logic zero state on the communication bus, it is enabled to detect a logic zero to logic one transition forming the start of the SOM header logic one state. A logic zero or logic one state is defined as present if at least three out of four consecutive readings of the bus (which occur at intervals of approximately once every 20 microseconds) are in a logic zero or logic one state respectively. A &#34;sample&#34; therefore represents the results of four consecutive readings. This technique adds significant noise immunity to the method. A logic zero to logic one transition at the beginning of a start of message header or at  the beginning of a data &#34;0&#34; bit or data &#34;1&#34; bit requires four consecutive readings to be as follows: logic zero, logic one, logic one, and logic one. The logic one to logic zero transition of a start of message header requires four consecutive readings to be as follows: logic one, logic zero, logic zero, and logic zero. This technique insures an accurate determination of the entire duration of the logic one state of the start of message header. 
     Thus after transmitting its initial message to the remote unit, the communicator waits approximately 20 ms to begin its search for this 1 ms preamble condition associated with the beginning of a start of message header. This is performed by the communicator taking continuous consecutive readings of the communication bus until a logic zero is found, followed by three logic one&#39;s which denotes the logic zero to logic one transition at the beginning of a start of message header (see reference area 67). The consecutive reading rate of the communicator is approximately once every 20 microseconds and thus the logic zero to logic one transition is nominally detected as an actual transition by the communicator within approximately 80 microseconds. 
     If such a logic zero to logic one transition is not detected within a window of 30 ms after the communicator starts sampling the line (that is after the 20 ms wait following the communicator&#39;s initial transmission to the remote unit), the communicator stops its search for a logic zero to logic one transition and treats this condition as a &#34;no reply&#34; condition. These conditions are discussed more fully below. 
     After finding a logic zero to logic one transition marking the beginning of a start of message header, the communicator takes samples of the input for the first fourteen nominal tick periods. All of these smaples (each comprising four consecutive readings as discussed above) must result in logic one state for a valid start of message header to considered present. In this situation, the communicator waits until the end of the fourteenth nominal tick time (that is for 8.75 ms) at which time the communicator takes continuous consecutive readings of the bus (lines 26&#39; and 26&#34; as defined above) so as to detect a logic one to logic zero transition indicating the end of the actual 28th tick of the incoming start of message header. Since the amount of time from the fourteen nominal tick time until a logic one to logic zero transition is accumulated by the communicator, this length of time is used in combination with the fourteen nominal tick times to determine the actual length of time which the start of message header was in the logic one state. 
     This length of time is equal to fourteen times 625 microseconds (the first fourteen nominal ticks) plus the length of time after the fourteenth nominal tick time in which a logic one state was sensed by the communicator (see FIG. 4A). This time can be calculated by the following equation. ##EQU1## 
     In the example shown in FIG. 4A, the logic one to logic zero transition occurred between the twenty-sixth and twenty-seventh nominal tick time indicating that the actual tick time for the remote unit sending the SOM header was shorter than 625 microseconds per tick time. For instance, if this measured time equals 7.7 ms, then according to the above equation, the actual tick time equals 8.75 ms plus 7.7 ms divided by 28, or 0.5875 ms (587.5 microseconds). 
     Once this calculation is determined by the communicator, it sets its tick timer to the actual tick time of the remote unit so that all subsequent references to tick times for determining a data &#34;1&#34; or a data &#34;0&#34; bit is sampled using this actual tick time. 
     In addition, in order to complete the test for a valid start of message header, it is necessary that the header maintain a logic zero state during at least two other samples of its actual 29th through 32nd tick times. These samples may be made at the 29th, 30th, and 31st actual tick times and if both of these samples indicate a logic zero state, then a valid start of message header is considered found and the actual tick time determined. 
     Program Execution of Transmission Rate Detection 
     The above determination of the remote unit&#39;s actual transmission rate is performed by central processing unit 58 shown in FIG. 2A under program control. 
     The overall computer program stored in memory 56 (see FIG. 2C) and executed by central processing unit (CPU) 58, a 6805 type CPU, is set forth in Table 2. Table 2 is written in assembly language and the portion regarding receipt of an overall message having a format as shown in FIG. 7 is given from page 3, line 1,000 through line 8490. This portion of the program includes the determination of the actual tick time. 
     The routine entitled BAUD.START (lines 2710 through 3420) performs the search for a logic zero to logic one transition which must be found to determine the presence of a SOM header, as well as to restart the tick times for determining the presence of a logic &#34;0&#34; bit or a logic &#34;1&#34; bit. 
     Once a logic zero to logic one transition is found, the SOM routine is invoked (see line 1770) which appears at lines 4840 through 7040. This routine checks to make sure that the first fourteen nominal ticks are logic one&#39;s by invoking the SAMPLE routine (lines 4120 through 4820). The SAMPLE routine takes four consecutive readings of the bus and requires that at least three out of the four readings either be in a logic one state or a logic zero state for that state to be considered valid. This routine forms a digital filter which tends to suppress spurious voltages on the bus from being considered valid logic levels. This routine is also used for testing logic levels associated with data &#34;0&#34; bits and data &#34;1&#34; bits. 
     If logic one&#39;s are obtained for the first fourteen nominal ticks, the routine looks for a logic one to logic zero transition and keeps track of the elapsed time while looking for this transition. Due to limitations of the 6805 CPU, a PRESCALE register is used to keep track of some of the elapsed time (see lines 5180 through 5800). The calculation of the actual tick time is presented from lines 5820 through 7040. 
     To insure a valid SOM header, the main routine at line 1790 calls the MARK.END routine (lines 7210 through 7360) to make sure that logic zero is detected for the two sampled ticks between actual ticks 29 through 32. 
     The routine for detecting data &#34;0&#34; bits and data &#34;1&#34; bits is entitled DATA.BIT at lines 3440 through 4090. This routine uses the BAUD.START routine to test for a logic zero to logic one transition. If found, it resets the actual tick timer so as to avoid the build-up of cumulative timing errors which otherwise could result due to difference between the calculated actual tick time and that produced by the remote unit, as well as variations which might occur in the remote unit&#39;s transmission rate. 
     The DATA.BIT routine then samples the logic levels at tick times 1, 2, 3 and 4, requiring that each determined logic level be logic one&#39;s for a valid data bit (see FIGS. 5 and 6). It then samples the logic level at tick time 8 to determine whether the data bit as a &#34;0&#34; or &#34;1&#34; (see FIGS. 5 and 6). Each sample uses the SAMPLE routine. 
     The BAUD.END routine is then invoked to insure that the data bit is valid. In this way, high nose immunity is obtained in the detection of each data bit comprising the incoming message shown in FIG. 7. 
     The parity is incremented and checked at lines 2030 through 2470 while the EOM routine insures a proper end of message (two data &#34;0&#34; bits) at the end of the fourteen bit (baud) message (see FIG. 7 and lines 7510 through 7930). 
     The remaining portion of Table 2 implements all the other functions of the communicator including interrogating the remote units and causing printer output to associated printer 60 (FIG. 2C) for receipt of the dot outputs from IC 7 (see FIG. 2E) and from IC 9 (see FIG. 2C). 
     Although the above procedure uses the determination of a tick time, the procedure could equally have measured the logic one state of the SOM over its entire duration without reference to such tick times. 
     Since each overall transmission comprises a start of message header, eight address bits, five status bits, a parity bit and an end of message header (see FIG. 7), an incoming address bit immediately follows the start of message header from the remote unit. Thus the communicator continues to sample incoming data so as to determine data &#34;0&#34; bits and data &#34;1&#34; bits with a sampling technique for both as shown in FIGS. 5 and 6. As there seen and as explained above, the communicator samples at actual tick times, 1, 2, 3 and 4, followed by a sample at tick times 8, 14, 15 and 16. Since both the data &#34;0&#34; bit and data &#34;1&#34; bit should be in the logic one state for actual tick times 1, 2, 3 and 4, it is required that in order to be considered valid data, the first samples taken at tick times 1, 2, 3 and 4 must always be in the logic one state. Similarly, as shown in FIGS. 5 and 6, both a data bit &#34;1&#34; and a data bit &#34;0&#34; must be in a logic zero state during tick times 13 through 16 and therefore the samples taken at actual tick times 14, 15 and 16 must always be in a logic zero state in order for the data to be considered valid. 
     It is important to note that the communicator resets the tick time to zero only when it senses a logic zero to logic one transition as discussed above; that is, when four consecutive readings are logic zero, logic one, logic one and logic one. By resetting its tick timer at the start of each logic zero to logic one transition (which transition is present at the start of each data bit), any error in the determined actual tick time is prevented from accumulating during a transmission. It also insures that the sample taken at tick 8 is always going to occur at the mid-region of the data bit. It is this mid-region that denotes the nature of the data bit (whether it is a &#34;0&#34; or &#34;1&#34;). 
     The sample taken at tick time 8 then accurately determines whether the incoming data bit is a data bit &#34;1&#34; or a data bit &#34;0&#34;. If the sample taken at the 8th tick time is in a logic one state, then the data bit is a data bit &#34;1&#34;, and conversely if the sample is in a logic zero state, the data bit is a data &#34;0&#34; bit. 
     Since the message obtained by the communicator is with regard to a particular addressed remote unit, the first eight data bits following the start of message header represent the address of the remote unit and must correspond to the address which was sent out by the communicator. If a match is not obtained (that is, if the received address is different from the transmitted address) then the communicator causes a message to be printed on an associated printer 60 (see FIG. 2C), indicating that a wrong remote unit answered the communicator. 
     If the address does correspond to the transmitted address, the communicator causes a message to be printed on its associated printer indicating the meaning of the next five status bits according to Table 1. 
     
                       TABLE l                                                     
______________________________________                                    
Status Bits    Command/                                                   
S0   S1     S2    S3   S4  Response Meaning                               
______________________________________                                    
0    0      0     1    1   C        start 5 min. self-test                
0    0      0     0    1   C        start 30 min. self-test               
0    0      1     0    0   R        self-test started                     
0    0      0     0    0   R        not started                           
0    0      0     1    0   C        report status                         
1    X      X     X    X   R        charger: fail                         
X    X      1     1    X   R        battery: fail                         
X    X      1     0    X   R        transfer: fail                        
X    X      0     1    X   R        lamps: fail                           
X    X      X     X    1   R        electrolyte: fail                     
0    1      0     0    0   R        status is normal                      
0    0      0     0    0   R        status is normal                      
______________________________________                                    
 X = &#34;Don&#39;t care                                                          
 C = indicates transmission from the host communicator 22                 
 R = indicates transmission from a remote unit 24                         
 
    
     As also seen in Table 1, the communicator can command various actions of the remote unit, including the command to cause the remote unit to report its status, a command for the remote unit to start a five minute self-test, and a command to cause the remote unit to start a thirty minute self-test. These commands as well as the selection of which units to be interrogated is controlled by the user through keypad 52 shown in FIG. 2D. The program implementations of these functions is also presented in Table 2. 
     SWITCH FUNCTIONS 
     The switch functions of keypad 52 are as follows: 
     KEYS O-F: These keys are used to address the remote units to be polled by the communicator for a one unit scan, or the starting address of the first remote unit to be polled if all units have been selected. 
     FEED: This key advances the paper associated with the interconnected printer to facilitate paper loading and also leaves a margin at the end of a report. 
     QUIT: This key stops the scanning process if all units have been selected. If pressed prior to the start of a scan, it will return to the status/test selection mode. 
     30 MIN: This command causes the remote unit to initiate a thirty minute self-discharge test. 
     5 MIN: This command causes the remote unit to initiate a five minute self-discharge test. 
     STATUS: This command causes the remote unit to report its status. 
     ALL: This key selects all units for either a status report or a self-test initiation. 
     ONE: This key selects only one unit for a status report or a self-test initiation. 
     ENTER: This key causes the communicator to accept the last two entries on keys O-F as the starting address for an ALL units scan or the address of a single unit on a ONE unit scan. When this key is pressed a second time, it causes the communicator to enter into its transmit mode. Upon power up, the host communicator defaults to an ALL units STATUS scan. This scan is initiated by pressing the ENTER switch four times. 
     INDICATORS: Light emitting diodes are located under the ENTER, ONE, ALL, STATUS, 5 MIN and the 30 MIN keys. These light emitting diodes 54 are shown in FIG. 2E and illuminate and blink at various times under control of the controlled program set forth in Table 2. The blinking guides the user in the operation of the communicator. 
     The examples numbered one through seven in Table 3 indicate the various tests and results obtained using the communicator according to the present invention. 
     OPERATION MODES 
     The operation modes to obtain the various reports shown in Table 3 allow the communicator to cause remote emergency lighting units to do one of three things: namely, to report their status, to start a five minute self-discharge test, or to start a thirty minute self-discharge test. 
     
                       TABLE 3                                                     
______________________________________                                    
EXAMPLE #1                                                                
5 MINUTE TEST                                                             
SCAN ALL UNITS STARTING AT                                                
UNIT:00                                                                   
UNIT:00                                                                   
TEST STARTED                                                              
UNIT 02:                                                                  
TEST STARTED                                                              
 UNIT:04                                                                  
TEST STARTED                                                              
UNIT:06                                                                   
TEST STARTED                                                              
UNIT:08                                                                   
TEST STARTED                                                              
UNIT:9F                                                                   
TEST STARTED                                                              
UNIT:20                                                                   
TEST STARTED                                                              
UNIT:33                                                                   
TEST STARTED                                                              
UNIT:3F                                                                   
TEST STARTED                                                              
UNIT:FF                                                                   
TEST STARTED                                                              
ENDING AT                                                                 
UNIT:FF                                                                   
DATE                                                                      
ACCEPTED BY:                                                              
EXAMPLE #2                                                                
30 MINUTE TEST                                                            
SCAN ONE UNIT                                                             
UNIT:FF                                                                   
TEST STARTED                                                              
DATE                                                                      
ACCEPTED BY:                                                              
EXAMPLE #3                                                                
5 MINUTE TEST                                                             
SCAN ONE UNIT                                                             
UNIT:4C                                                                   
NO REPLY                                                                  
DATE                                                                      
ACCEPTED BY:                                                              
EXAMPLE #4                                                                
30 MINUTE TEST                                                            
SCAN ONE UNIT                                                             
UNIT:A2                                                                   
NOT STARTED                                                               
DATE                                                                      
ACCEPTED BY:                                                              
EXAMPLE #5                                                                
STATUS REPORT                                                             
SCAN ALL UNITS                                                            
STARTING AT                                                               
UNIT:00                                                                   
UNIT:00                                                                   
STATUS IS NORMAL                                                          
UNIT:02                                                                   
STATUS IS NORMAL                                                          
UNIT:04                                                                   
STATUS IS NORMAL                                                          
UNIT:06                                                                   
STATUS IS NORMAL                                                          
UNIT:08                                                                   
STATUS IS NORMAL                                                          
 UNIT:0F                                                                  
STATUS IS NORMAL                                                          
UNIT:20                                                                   
STATUS IS NORMAL                                                          
UNIT:33                                                                   
=CURRENT STATUS=                                                          
 CHANGES: PASS                                                            
=PER LAST TEST=                                                           
 BATTERY: PASS                                                            
 TRANSFER: PASS                                                           
 LAMPS: FAIL                                                              
UNIT:3F                                                                   
STATUS IS NORMAL                                                          
UNIT:FF                                                                   
STATUS IS NORMAL                                                          
ENDING AT                                                                 
UNIT:FF                                                                   
DATE                                                                      
ACCEPTED BY:                                                              
EXAMPLE #6                                                                
STATUS REPORT                                                             
SCAN ONE UNIT                                                             
UNIT:3F                                                                   
STATUS IS NORMAL                                                          
DATE                                                                      
ACCEPTED BY:                                                              
EXAMPLE #7                                                                
STATUS REPORT                                                             
SCAN ONE UNIT                                                             
UNIT:5E                                                                   
NO REPLY                                                                  
DATE                                                                      
ACCEPTED BY:                                                              
______________________________________                                    
 
    
     STATUS REPORT 
     Selecting the STATUS key on keypad 52 causes the communicator to enter into the status report mode. In this mode, a command is issued which causes the remote unit to report its status. Status indicates the current status of the unit including its charge state and electrolyte level for remote emergency lighting units such as those disclosed in application, Ser. No. 07/048,530, filed May 8, 1987. It also causes results of the last-self discharge test to be transmitted, including the battery condition, the transfer circuitry condition and the lamp condition. If there are no failures, a &#34;status is normal&#34; message is printed on the associated printer. If there are any failures, then a complete status report is printed. 
     ONE UNIT SCAN 
     After the user enters the address of the unit to be polled by pressing the ENTER key, the communicator initiates the scan of that unit. After receiving the reply from the remote unit, the communicator prints either &#34;status is normal&#34; or a failure report. Formats given in examples 5 and 6 of Table 3 are representative of these reports. If the unit does not answer, such as due to a communication failure or that no unit is at the selected address, then a &#34;no reply&#34; message is printed along with the unit address. This type of result is shown in example 7 of Table 3. 
     ALL UNITS SCAN 
     This type of scan is started after entering the address of the first unit to be polled and pressing the ENTER key again. After receiving the reply, the communicator prints the report and the unit addresses and then polls the next consecutive address until all addresses have been polled or until the QUIT key is pressed. During such an ALL units scan, the ALL LED flashes to inform the operator that the communicator is in such a mode. 
     TEST MODE 
     The test mode initiates either a five minute or thirty minute self-discharge test. This mode only initiates the self-test. To obtain the results of the tests, a STATUS report must be requested after completion of the test. 
     ONE UNIT SCAN 
     After entering the address of the remote unit to be tested, the ENTER key is pressed a second time to initiate the self-test command. Three possible responses may be printed in response to a ONE unit self-test scan: namely, no reply, self-test started, or not started. (See examples 1, 2, 3 and 4 of Table 3). A NO REPLY and the unit address is printed if the remote unit does not respond or there is no active unit at that address. If the remote unit does not start the self-discharge test, then &#34;NOT STARTED&#34; along with the unit address is printed. The messge TEST STARTED is printed along with the unit address when the remote unit enters a self-test. 
     ALL UNITS SCAN 
     After entering the address of the first remote unit to be polled and pressing the ENTER key a second time, the communicator begins to poll each consecutive address starting with the one entered, until all addresses have been polled or the QUIT key is depressed. If the remote unit does not answer or there is no active unit at that address, nothing is printed. As each remote unit answers, the message SELF-TEST along with the unit address is printed. 
     Thus, what has been described is a communication method and system which can accommodate a wide range of transmission speeds of a plurality of remote units by determining the transmission speed through sampling of a start of message header so as to determine the actual tick time of the remote unit during receipt of its start of message header. In addition, the present invention incorporates a communicator which can initiate a number of tests to one or more interconnected remote units, specifically with regard to such remote units operating as part of an emergency lighting system. 
     It will thus be seen that the object set forth above, and those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in carrying out the above method as well as the described system, without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
     It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be set to fall therebetween. ##SPC1##