RS-485 'polarity'

[ltj]

I'm having problem with the A and B designation on RS-485 driver chips.
Between some chips I have to hook up A to A to get it to work, while on
others I have to reverse it and hook A to B. I've double checked the spec
sheets and am using the A and B for the pins that the manufacturer
specifies.

confused,
ltj

 

[CFoley1064]

Subject: RS-485 'polarity'
From: "ltj" ltj@yahoo.com
Date: 3/19/2004 11:38 AM Central Standard Time
Message-id: <405b3001$0$73110$a0465688@nnrp.fuse.net

I'm having problem with the A and B designation on RS-485 driver chips.
Between some chips I have to hook up A to A to get it to work, while on
others I have to reverse it and hook A to B. I've double checked the spec
sheets and am using the A and B for the pins that the manufacturer
specifies.

confused,
ltj

RS-485 is a method of "Balanced Data Transmission" using twisted pair wire.
According to the standard, when a signal is active, A will be positive relative
to B. When the signal is inactive, B will be positive relative to A.

In fact, what you're doing is what most people do -- hook it up, see if it
works, and if not, just reverse it.

Good luck
Chris