B&B Electronics 485T - Manual (Page 12)

10 Documentation Number 485T1995 Manual
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 433-5105
See B&B Electronics' free RS-422/485 Application Note for
information on proper wiring and grounding of an RS-422/485
network.
If the POSITIVE and NEGATIVE THRESHOLD LEDs are both
off, you can then test for proper signal amplitude. With the
THRESHOLD pot turned to zero (full clockwise), the DATA LED
should blink or stay on when data is sent. When data is being sent
on the network, turn the THRESHOLD pot counterclockwise until the
LED goes out.
If, for instance, the LED goes out when the pot is pointing at .5,
then you have a differential input voltage of plus or minus .5 volts.
Both RS-422 and RS-485 require at least plus or minus .2 volts to
work properly. If your network node is below this level, you may
need a line booster, a repeater, or you may need to change your
wiring layout. Refer to B&B Electronics' free RS-422/485 Application
Note for information on network wiring. If the LED does not go out
with the THRESHOLD pot turned up to 1.0 (full counterclockwise),
then your differential input voltage is more than 1 volt. Most RS-
422/485 drivers start out with a differential of about 2 volts but the
resistance, inductance, and capacitance in the line can reduce it
considerably.
NOTE: The differential voltage you are trying to measure is the
RECEIVED voltage. You should only make the threshold
measurement when the node you are testing is RECEIVING
data, not when it is transmitting. If you have a multiple node
system with each node at a different location, you may have to
have each node transmit separately while you take a reading.
You may get a different differential voltage reading from each
node.
For example, if you have an RS-485 network with five nodes
each located 1000 feet apart for a total of 4000 feet. Node 1 is at
one end, node 2 next, etc., and node 5 is at the far end. If you
connect the 485T Tester to node 1 and watch while data is sent from
node 2, there will only be 1000 feet of wire and the signal should be
fairly strong.
However, when node 5 sends data, there will be 4000 feet of
wire and the signal will be weaker. If you have the different nodes
sending randomly it will be impossible to take a reading. You have
to have only one node sending to take a proper reading. You may
be able to turn the power off unwanted nodes to stop them from
transmitting. However, this may change the loading on the line, so
be careful using this method.