Datum bc632D, User Manual

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Datum Inc bc632D Wall Mount LED Display (Rev A) B-1
APPENDIX B
bc632D CONTROL BUS PROTOCOL
B.0 INTRODUCTION
Receiving devices can read the time message to keep their clocks synchronized to within one second.
To adjust to tighter synchronization, the receivers must adjust their clocks to the true on-time mark using
the time tags.
Each message has a reference point. For digital time, the reference point is the stop bit of the first
asterisk, or the trailing edge of the 8 uS reference pulse. For multiplex time, the reference point is the
stop bit of the first “C” of the "CHN" message.
The time tags specify the number of half-microsecond units between the master's on-time mark and the
reference point. For example, a value of "0007D0" indicates that the stop bit of the most recent
reference character was sent at 1000 microseconds after the on-time mark (7D0 hex = 2000 decimal,
2000 * 0.5 microseconds = 1000 microseconds).
Figure B-1
IBM PC Compatible Connection
IBM PC Compatible bc632D J3-RX
RS-232 25-pin female RJ11
Pin Pin
2 1 RS-232 IN
1 2 GND
3 6 RS-232
Because both transmitters and receivers are connected to the same pair of lines, all characters
transmitted by the computer will be echoed back to the receiver. This is a good way to determine
whether or not the physical connection has been made correctly.
B.1 RS-485 SIGNALS
The signals carried between bc632D's are RS-485 compatible. The two RJ11 jacks on each display
are wired together so that standard telephone cable can be used between devices. Standard telephone
cables are wired as shown in Figure B-2. Notice that signals get swapped between pins 1 and 6; 2 and
5; and 3 and 4 from one end of the cable to the other. The wiring inside the displays swaps the signals
back to the correct pins to continue the daisy-chain. Because of the internal swapping, the control bus
will not operate properly with cable that is wired pin 1 to pin 1.