Talk:RS-485

Merge request

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)

Huh?

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.

2-Wire and 4-Wire Implementations

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.

More polarity confusion

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.

A signal shown in blue, B in red

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)

Polarity per RS485 IC manufactures

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)