B&B Electronics labels the data lines on RS-422 and RS-485 with an "A", for negative or "B" for positive, to indicate the positive and negative relationship between the two data lines. It is important to label the data lines because this positive and negative relationship between the lines must be maintained when connecting one device to another.
Some manufacturers of RS-422/485 equipment label the data lines on their devices with "+" or "-", Signal and Inverted Signal, or an "A" and "B" that are opposite to other converters or devices in the network. This can add confusion when making connections or troubleshooting a network with devices from different manufacturers. This application note addresses some of these differences, show how they've come about, and try to relate them to each other so that the system designer can interconnect devices from different manufacturers.
The RS-422 and RS-485 Standards define the two differential signal lines as the "A" and "B" line. The signal state (0 or 1) is defined as the difference in voltage between the two lines. At any receiver, a "1", (Idle, Mark, or Stop bit), state is defined when the voltage on the "B" line is greater than the voltage on the "A" line by at least 200 mV. A "0" (Space or Start bit), state is defined when the voltage on the "A" line is greater than the voltage on the "B" line by at least 200mV. Shown in the equations below:
Va - Vb < -0.2V = "1"
Regardless of the naming scheme used, the main consideration in any RS-422/485 system is that the inversions remain constant throughout the system. If we relate the above states to standard TTL logic, when the "A" line is greater than the "B" line, this would equal a 0 Volt TTL level. Conversely, when the "B" line is greater than the "A" line, this would equal a 5 Volt TTL level. Figure 1 shows a typical asynchronous data byte and it's corresponding signal levels in TTL, RS-232, and RS-422/485.
"A" and "B" are opposite of what is shown in the standard.
The signals are labeled "+" and "-"
The signals are labeled "Signal" and "Signal*" or "Inverted Signal"
B&B tries to include the RS-422/485 specifications' naming scheme on all of our products. Because of its common use, most of our data sheets and product labels also include the "+" and "-" naming scheme along with the "A" and "B".
Reversing the two lines in the differential signal is by far the most common initial error when connecting two pieces of RS-422 orRS-485 equipment. The good news is that it won't cause any damage to your equipment to try it the wrong way. If you try it connected one way and are seeing garbled data, reverse the leads and try again.
If this doesn't instill a level of confidence you are comfortable with, or you have tried both combinations with negative results, you can usually use a DC voltmeter to determine which line is which.