EDWARDS SYSTEMS TECHNOLOGY IRC-3 Maintenance And Troubleshooting Manual page 12

NOTE 1
24 V
5 V
0 V
ZAS COMMUNICATIONS
Figure 2
6
maximum of 26.4 VDC to a lower voltage, for example 23 VDC.
The 3.4 volt drop in the wiring is due to wire resistance.
To measure ZAS line voltage drop, use an oscilloscope to measure
the peak voltage at the ZAS module and at each analog/addressable
device. If the voltage difference is greater than 2 VDC, the
resistance in the wire run is excessive. Too much resistance in the
ZAS wire run is typically caused by small wire size or a bad
connection.
If the wire size is too small for the run length, the only remedies are
to replace the wire with a larger size or install additional ZAS
modules, dividing the line into acceptable lengths. Breaks or bad
connections in the ZAS loop wiring can be identified by comparing
the calculated loop resistance value (described earlier) with the
measured loop resistance value. The measured wiring loop
resistance should not be different from the calculated loop resistance
by much more than a few ohms.
2. Excessive Wiring Capacitance:
The second cause of erratic ZAS line operation is too much
capacitance in the ZAS circuit wiring. Capacitance distorts the
digital signal. Compare the waveform in Figure 1 with that of Figure
2. The capacitance causes the distortion shown in Figure 2 (note 1).
As wiring capacitance increases, the square edges of the digital
wave form start to curve. Excessive wiring capacitance causes the
waveform to curve beyond the point where a device can recognize
the waveform and respond when polled.
NOTE 2
ANALOG / ADDRESSABLE DEVICE COMMUNICATIONS
Wiring capacitance also effects the turn on current spike shown as
note 2 in Figure 2. If the turn on current spike is not present in the
digital sequence, there is a high probability the analog/addressable
device's communication will not be understood by the ZAS
communication module.
To determine if excessive capacitance is the cause of ZAS line
problems, write and download into the CM1/2(N) which contains
the problem ZAS card, a short program which communicates with
only one device on the ZAS line. Disabling all but one device on the
ZAS line will not accomplish the required task.
With all devices connected to the ZAS line and the single device
program loaded, use the oscilloscope to verify the observed
waveform on the terminals of the devices is similar in appearance to
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Maintenance and Troubleshooting Manual