Troubleshooting, continued
80
Problem Diagnostics indicate there is a fault with the field decoders.
Solution There is a break in the two-wire communications cable, the connections are loose, the decoders
are supplying too much or too little voltage, or the controller is defective. To check the line
voltage and decoders, you will need a voltmeter and Clamp Ampmeter.
Navigate the MDC Controller to the Short Finding Screen (this switches the controller to
the 50 Hz mode). Please see Finding a short in the system on page 40. This enables you to
measure line voltage (in the AC mode) and currents with the Clamp Ampmeter.
Start by measuring the line voltage between L1 and L2 on the terminal strip. Nominal voltage
is 34 V, but because the voltage form is non-standard (standard=sine), your reading will
probably be higher. Acceptable values are from 33 – 40 V.
If the value is too low, disconnect the line wires (L1 and L2) and measure again. If the voltage
is still too low, the controller is defective and you will need to replace it.
If the voltage is within the acceptable range, there is a problem with the field installation.
Please see the problem, The field decoders are not responding on page 81.
With the lines (L1 and L2) connected, use the Clamp Ampmeter to measure the line current.
Standby current for the FD-102TURF and FD-202TURF Decoder is 0.5 mA.
Standby current for the FD-401TURF and FD-601TURF Decoder is 1 mA. Based on these
values, compute the expected line current for your system. If the actual current differs more
than 20 mA from the value you computed, the problem is in the field.
Voltages or current readings that are too high indicate that something is overloading the
two-wire communications cable. Please see the problem, Overloading the two-wire
communications cable caused a short circuit on page 84.
Current reading too low may indicate that there is a break in the two-wire communications
cable. Please see the problem, Finding a break in the two-wire communications cable on
page 83.
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