Chapter 7: Service and troubleshooting
EST3X Technical Reference Manual 225
Analog data loop wiring problems
There are three basic wire-related analog addressable loop problems: excessive
resistance, excessive capacitance, a ground faults.
1. Excessive wiring resistance: Rarely is excessive wiring resistance the sole
cause of analog addressable loop problems. For any length of cable, the
amount of resistance and capacitance per foot does not change and the
analog addressable circuit capacitance limits are usually reached before the
resistance limits. The signal operates between 0 and 24 VDC. Excessive loop
resistance causes the signal to shrink from a maximum of 23 VDC to a lower
voltage, for example 20 VDC. The 3 V drop in the wiring is due to wire
resistance.
To measure the analog addressable loop voltage drop, use an oscilloscope to
measure the peak voltage at the analog addressable 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 analog
addressable 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 analog addressable modules,
dividing the loop into acceptable lengths. Breaks or bad connections in the
analog addressable loop wiring can be identified by comparing the calculated
loop resistance value 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: Excessive capacitance in analog addressable
loop wiring will distort the digital signal. As wiring capacitance increases, the
square edges of the digital waveform 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.
Wiring capacitance also affects the turn-on current spike. 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
3-AADC1 controller.
Analog addressable circuit capacitance problems are typically caused by long
wire runs, ground faults on the loop, improper T-taps, or improper shielding.
If shielded wire is used, the shield must be treated as a third conductor. It
must be free of all ground faults and have continuity throughout. If the wire
capacitance is too large for the run length, the only remedies are to replace
the wire with a cable having a lower capacitance per foot or to install
To Next Page
Loading...
