RS485 A/B line reversal in extant devices

Every time I design a product with an RS485 interface, I wind up going over the same old ground in disbelief - and coming up with the same answer. The A/B line-naming convention is reversed when comparing extant devices (e.g. the venerable SN75176B, or the transceiver product line from Maxim) with the RS485 (ok, EIA485) specification.

Perhaps this confusion stems from the definitions and interpretation of MARK and SPACE. But even avoiding those entirely, I'm still left with the fact that the idle state of an RS485 line involves A being low and B being high. When using e.g. a Maxim part with failsafe inputs (which force the idle state when disconnected) I always have to cross the A/B lines to bring them out of the product. (With the 75176 I can either do that, or invert the Tx/Rd UART signals. The former is easier ;).)

Yet a doubt remains - surely there should be more noise about this issue than I've seen. Anybody feel that this a serious trap for the newbie who doesn't have the actual RS485 spec to hand?


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Steve at fivetrees
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This is indeed weird. When I first designed some RS485 devices, they were only meant to talk to eachother, so I just followed the datasheets of the MAX485 and equivalents.

But later, developing NMEA devices, I really thought that the NMEA standard defined RS-422, but with reversed A and B designators, since it has A negative from B in idle. But it turns out to be the correct way. The advantage of this is that you can sometimes connect RS-232 to RS-422 with success, which wouldn't be possible with A positive from B in idle. To add to the confusion, the EIA485 says that the A line can also be designated by a '-' and the B by a '+', while all NMEA devices, A is equivalent with '+' and B with '-'.....




It seems to me that the first manufacturer got it wrong and all others just followed blindly...


Reply to
Meindert Sprang



Which is fine - it'll work, but won't comply with the spec. Or indeed talk to anything that *does* comply with the spec ;).


Possible in one direction only - since one of the differential lines has to act as ground. FWIW, RS423 is the official halfway-house between RS232 and RS422/485 - basically RS232 with +/-5V swing rather than +/-12V.



Indeed. I've seen this confusion in a lot of different places and in a lot of documents from varied sources.



Yes, that's my view. The first time I came across this was when the original

75176A was first introduced (mid 80s); the company I was working for had already had a lot of experience with RS422 devices and we spotted the error fairly quickly. Seems that all the later devices have essentially copied the error from there.


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Steve at fivetrees

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