RS-422/485 TEST
A typical test on an RS-422/485 network might go like this: First start by locating the two wires used for the RS-422/485 network and try to determine which is A and which is B. Also, locate the Signal Ground. This may be the same as the ground on the DC power supply that powers the RS-422/485 device.
Connect the Signal Ground to the GND terminal. Connect the A and B terminals to what you think are the A and B wires. Turn the Threshold Pot to zero (full clockwise), the TERMINATION off, the RS-422 and RS-485 switch to the proper position, and the POWER switch ON. If you are not sure which is A and which is B, test them with a DC voltmeter. With no data being sent, the A line should be negative with respect to the B line. Getting the A and B lines correct is only important for the RS-232 DATA OUT CONNECTOR. It will have no affect on the other tests.
The POSITIVE and NEGATIVE THRESHOLD EXCEEDED LEDs should both be off, both when data is being sent and when the network is idle. If either of these LEDs is on or blinking, you may have a grounding problem. You should first check that the RS- 422/RS-485 switch is in the proper position and that the Signal Ground is connected correctly to the 485T. Refer to B&B Electronics' free RS-422/485 Application Note for information on proper grounding of your network.
NOTE: If the positive or negative thresholds are exceeded, you may damage your RS-422 or RS-485 line receivers. If either of the LEDs is on or blinking you should find out why and fix the problem as soon as possible.
For example, if you have a network with two nodes located in two different buildings about 4000 feet apart. An RS-422 driver in building A can be switching between +5 volts and ground and be powered from building A’s power lines. An RS-422 receiver can be located in building B with a ground potential that is 3 volts DC lower than building A. That receiver will see +8 volts for a high (5+3 volts) and +3 volts for a low (0+3 volts). This is allowed under both standards and should work fine. However, if the difference in ground potential is 10 VDC lower, the high (5+10 volts) will be +15 volts and is NOT allowed. The ground difference voltage can also be an AC voltage which could confuse things even more.
Documentation Number 485T1995 Manual | 9 |
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