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I am implementing a device which communicates on Profibus DP. In order to get the communication with my Siemens S700 PLC to work, I have to reverse the order of the 'A' and 'B' differential signal leads.

The profibus communication is implemented by a Profichip VPC3+S. I am using an ISO1176 RS485 which provides an 'A' and 'B' connection.

Any ideas why it is 'backwards'?

Houston Fortney
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2 Answers2

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This is apparently a common issue. A should be (-). It's more reliable to use (+) and (-) than A and B.

Here is a clip from a Siemens manual- they only show + and -.

enter image description here

Many devices can autodetect the polarity using one of a couple of possible methods, and correct automatically for reversal , however the autodetection can be fooled, for example if the terminators are not correctly set up to give the right idle level.

Edit: indeed Siemens (maybe Profibus) uses the opposite definition of A/B from most others. Use +/- and there should be less confusion.

enter image description here

Spehro Pefhany
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  • It is very plausible that Siemens uses different way of marking A and B, which could be the right opposite as many RS485 transceivers use. – Marko Buršič Feb 08 '16 at 19:03
  • @MarkoBuršič you are absolutely correct, see edit – Spehro Pefhany Feb 08 '16 at 19:23
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    Thanks guys. Seems odd to me that A is - and B is +, but the final answer is A,-,green B,+,red. The profibus definition is different than the generic RS485 nomenclature. – Houston Fortney Feb 08 '16 at 20:24
  • I wouldn't be surprised if Siemens *intentionally* created some ambiguity here. Open a mid-size G120 "power module" VFD and you'll see a 24V red and black wire coming from the PSU board to the driver board. Not only are these the wrong colors for 24V (brown and blue) but red is *zero volts* and black is *+24*. – rdtsc Jul 05 '23 at 12:53
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B - red wire, plus, DSUB9 pin3
A - green wire, minus, DSUB9 pin8

Maybe the cable is swapped on slave device. Do you use Siemens profibus connectors, if yes then it is almost impossible to wire them wrong, but I have seen also this scenario. Did you make the slave, if yes check that B is biased at Vcc and A is biased at gnd?
Here is one exaple where RS485 transceiver has AB swapped with respect to Siemens AB marking, so the plausible explanation is: Siemens vs Others = swapped. You know, in Simens averything is done a little bit complicated as it should be, if everywhere in the world use A and B, they use B and A just to make things more difficult.

http://www.nve.com/isoapps-PROFIBUS.php

Marko Buršič
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