User's Manual

Page 31

Ground Potential Difference Check

Before a device is connected to an RS-485 subsystem, it must be checked for

ground fault. An uncorrected ground fault can damage all devices connected

to the RS-485 communication line.

To check if there is ground fault for a new unit, follow the steps below:

1. Apply power to all devices already successfully connected to RS-485 line.

2. Power up a new unit, but DO NOT connect it to RS-485 line.

3. Connect the signal ground of the RS-485 line through a 10-kΩ current limiting

resistor to the signal ground (see Table 2) of the Veri-Series unit. There should

NOT be more than 1 volt across the resistor. Otherwise find and clear the

fault.

4. Repeat the steps in #3 with each of the RS-485 signal lines (+ and –)

5. Connect the new unit to the RS-485 line only if no ground fault is found.