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You need to know your ABC’s – Especially in RS-485

You need to know your ABC’s – Especially in RS-485

| 7/11/2013 2:24:22 PM

Whether you’re wiring up a controller for the first time or stuck trying to figure out why an installation that has been working fine for many years suddenly refuses to communicate, it helps to understand the data line and how it is labeled. This is important because the positive and negative relationship between the lines must be maintained.
 
We’vNeed to Know Your ABCse seen every data line labeling scheme. Some are better than others. Some seem deliberately deceptive. For example, one manufacturer may use a simple “+” and “-“. Others might spell it out; “Signal” and “Inverted Signal.” Many simply say “A” and “B.” This often confuses technicians.
 
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"
 Va - Vb > 0.2V = "0"
 Where Va and Vb are the voltages on the "A" and "B" lines respectively.
 
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.
 
Many times, manufactures get the A and B mixed up. This is due to the IC manufacturers labeling the inverted pin of a transceiver as B. The design engineer naturally assumes that the IC chip labeling corresponds to the RS-485 standard and carries the labeling through.
 
B&B includes the RS-422/485 specification naming scheme on all our products. Since it is very common in the industry, we also include the “+” and “-“. So you will see TDA(-), RDA(-), DATA B (+), etc. on listed on our datasheets and quick start guides.
 
Reversing the two lines in the differential signal is by far the most common initial error when connecting two pieces of RS-422 or RS-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.
 
1. Measure the voltage across the two lines at the receiver without the driver connected. On an RS-485 device the driver and receiver are on the same pair.
2. Note which signal line the positive lead of the voltmeter is connected to.
3. Is the reading positive or negative?
4. If the reading is positive, the signal line going to the positive lead of the voltmeter corresponds to the "B" line of the RS-422/485 standards.
5. If the reading is negative, the signal line going to the positive lead of the voltmeter corresponds to the "A" line of the RS-422/485 standards. 
 
 

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Abdul
You have raised very valid points in your post. It really happend some person do not have the right idea. But with your awesome tips one can easily identify it.
8/4/2013 12:56:05 PM

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