B&B Electronics manual A and B Terminals, Ground Terminal, CONTROLS AND CONNECTIONS ON THE 485T

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.

6Documentation Number 485T1995 Manual

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