Brasch GDCP-A, Инструкция по эксплуатации

Страница: 31 / 35

31
APPENDIX B
Description of 4-20 milliamps, multiplexed signal output.
The GDCP-A Control Panel can send an analog signal to external systems such as building
management systems, (BMS). This signal can be used by the BMS to trigger remote alarms,
data storage systems and the like.
The signal is sent over one 4-20 milliamp output cable. The cable is a two conductor, shielded
type, with the shield connected to the earth ground at one point only. Regardless of the number
of transmitters in the system, only one signal cable is required. The individual transmitter outputs
are time-multiplexed over this one cable. The output of the cable is monitored at the receiving
end, and the individual signals from each transmitter is determined by its time displacement from
a start pulse that is sent out on the same cable just before the signal from transmitter number one
is sent. Each transmitter’s signal follows, in sequence, at a preset time displacement, from the
previous signal. When all transmitter signals have been sent, the start pulse is again transmitted
and the entire signal sequence is repeated using the latest data.
The start pulse consists of a 2.0 milliamp level of four second duration. At the end of this four
second pulse, a two second duration pulse is transmitted at a level corresponding to the
concentration sensed by the first transmitter. This level will be between 4.0 milliamps, (0 PPM)
and 20 milliamps, (the transmitter’s full scale value). If the transmitter has failed, this level will be
at 0.0 milliamps.
As an example, if the transmitter is monitoring for carbon monoxide, and the concentration is at
35 PPM, the transmitted signal will be a two second pulse at 6.80 milliamps. At the end of the
two seconds, the signal from transmitter number one will be replaced with the signal from
transmitter number two. This signal will continue for two seconds, also.
In a system with six transmitters, the total signal time from the beginning of the start pulse to the
end of the sixth transmitter’s signal will be a total time of 16 seconds. This time frame will consist
of one four second start pulse followed by six, two second signal pulses. The four second start
pulse will then repeat to indicate to the receiving system that the current sequence has
completed, and the next sequence is beginning.
FIG. 9 shows the signals from this six transmitter system as a chart of signal level vs. time in
seconds.