B&B Electronics 485T - Manual (Page 11)

Documentation Number 485T1995 Manual 9
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 433-5105
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.