B&B Electronics 485T - Manual (Page 8)

6 Documentation Number 485T1995 Manual
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 433-5105
CONTROLS AND CONNECTIONS ON THE 485T
A and B Terminals
The two signal lines used in an RS-422 or RS-485 network
should be labeled A and B. These lines should be connected to the
A and B terminals of the 485T Tester. If your lines are not marked
(or are marked using another method) you can use a DC voltmeter
to determine which is which. With no data being sent, the A line will
be negative with respect to the B line. On an RS-485 network, if all
of the drivers are off, the voltmeter will read zero volts. Under these
conditions, it is not possible to use a voltmeter to determine which
line is A and which is B. Having them connected backwards will only
be a problem when using the RS-232 Data Out Connector.
Some networks will use four wires instead of two. In these
networks, one pair will be used to send data out from a master
device. The other pair will send data back from one or a multitude of
other devices. These networks can be tested using the 485T by
treating them as two separate networks. You would first run your
tests on one pair of signal lines and then run them again on the
other pair.
NOTE: Make sure that you are connected to the correct two
lines when using the tester. It is possible for the tester to look
like it is working properly when one line is connected and the
other is floating or connected to a wire in another pair.
Ground Terminal
The GND terminal on the 485T Tester should be connected to
Signal Ground for the device under test. The Signal Ground is
usually the same as the power supply ground for your unit. Some
systems will also have a Frame Ground that is usually connected to
the “Green Wire Ground” of your power supply AC plug. Frame
Ground can also be connected to the wire shield if your pair of wires
is shielded. You want Signal Ground not Frame Ground. In some
systems, Frame Ground will be connected to Signal Ground, usually
through a 100 ohm one-half watt resistor. You still want to be
connected to Signal Ground to make proper measurements, since
they may be at different voltages. There is a 100 ohm one-half watt
resistor connected internally between the GND terminal and the
internal Signal Ground (power supply ground) of the 485T. This is
the optional grounding method from both the RS-422 and the RS-
485 Standard.