Talk:RS-485

I'm requesting that this article have its history merged into EIA-485, where this article now is. (Someone else has improperly cut-and-pasted the article instead of moving it.) I made the request on the Helpdesk but got no response.   –radiojon 02:00, 2005 Jan 31 (UTC)

This seems like a nonsense sentence ... "between the ribbon cable?

This allows EIA-485 to implement linear topologies using only two lines and between the ribbon cable.

Might be good to discuss the differences of a 2-wire (half-duplex) over a 4-wire (full-duplex) implementation. This seems to have become an increasingly popular marketing point with various vendors.

The diagram labels the two signals U+ and U-. I have added a caption and updated the text in the section to clarify that U- = A and U+ = B. If someone wants to update the diagram replacing U- and U+ with A and B respectively so we have consistent signal descriptions in the article, that would be good.

Before correcting me on this, please read previous discussion above and RS-485#Signals. ~Kvng (talk) 17:22, 11 October 2019 (UTC)

Hi Kvng. I believe that the section RS-485#Signals is actually incorrect. If you are available to discuss I am happy to do so. My logic is as follows:

• Understanding that a microcontroller UART outputs a logic 1 when 'idle' (mark), and a start bit is a logic 0.
• ON THE WIRE, the sense of 1 and 0 is reversed. i.e. "UART Logic 1" == "Idle" == "Mark" == "ON" == "Binary 0 'on-the-wire'"
• Therefore in the diagram U+ should be marked A, and not B as you proposed.
• Closest reference, without buying the TIA-485 specification, is this TI document that has an excerpt of the TIA-485 specification — Preceding unsigned comment added by Edward Hague (talk • contribs) 14:28, 18 September 2023 (UTC)

My guess of why RS485 IC manufactures describes "A" to be positive because that side is pulled up by a bias resistor, and "B" to be negative because that side is pulled down by a bias resistor. Its the bias resistor pull direction, not the polarity of the data bit.

Bias Resistors (if exist):

• in the following, the "+" pins are pulled up.
• in the following, the "-" pins are pulled down.

The following is what I use in the custom symbols in my schematics. See page 10 of XR33052 datasheet.

RS485 ICs: (the dual letters come from the above datasheet and the merging of the letters from two RS422 buses)

• +A/Y = pin6 of 8pin RS485 ICs
• -B/Z = pin7 of 8pin RS485 ICs

RS422 ICs:

• +A = receiver pin
• -B = receiver pin
• +Y = transmitter pin
• -Z = transmitter pin

• Sbmeirow • Talk • 21:10, 10 February 2025 (UTC)

In the Signals section, I find it very confusing and unhelpful to discuss signal states in terms of "marks" and "spaces" and "on" and "off". Is there some reason this can't be explained much more simply without involving these terms? IC datasheets typically refer to the signals simply as "inverted" and "non-inverted"; there is no mention of "marks", "spaces", "on" or "off". At the end of the day, the purpose of a paired driver/receiver is to reliably convey a generic single-ended signal. I've used them for many apps that have nothing to do with marks or spaces, including conveying V/F and incremental encoder signals, and interfacing instruments to a parallel data highway. As an aside, the discussions about A and B also seem overly complicated and, in some cases, possibly incorrect. Lambtron talk 20:41, 30 May 2025 (UTC)