# Wiring up RS485



## msplsh (Jun 23, 2022)

Anybody know how to do this?  I've got a device with A, B, G and the USB adapter I'm going to use is GND, R-, R+,T/D-, T/D+

My current guess is:
What's A & B?
A Negative -
B Positive +
G Ground

4-Wire to 2-Wire Conversion
Short TXD+ and RXD+
Short TXD- and RXD-

End to end matchups
Leader        Follower
TX+        =>    RX+
TX-        =>    RX-
RX+        =>    TX+
RX-        =>    TX-

_Therefore connect...._
RX -    => A
RX +    => B
TX/D -    => A
TX/D +    => B


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## jbo (Jun 23, 2022)

May I ask what exactly your question is?

From the information given, it would seem like your device presents an unbalanced interface whereas your adapter expects a balanced interface. Those can be converted by either purchasing a corresponding COTS part or doing it yourself (depending on your electronics skills/tools/parts available).

That being said, a lot of those USB adapters tend to be able to handle both balanced and unbalanced signals. The good ones usually have some sort of selector switch for this (often good old DIP switches). Cheap ones offering this feature often try to auto-detect (and often fail horribly).

On top of this, there are devices in the wild which "speak" RS-485 but only provide logic-level signals (which is different from just unbalanced signals). Usually™ these don't feature D-Sub connectors to "prevent damage".
If that is the case, the next "hurdle" is to determine the expected logic levels. "Common" ones are 5VDC, 3.3VDC and 1.8VDC.

Getting this wrong can result in damaging the device's interface.
Can you provide a bit more information - especially regarding your device?


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## obsigna (Jun 23, 2022)

This is not a definitive answer. Whether you connect 2 or 4 wires is device dependent.

I connected devices to a RS485 bus in 2 wire mode. The device manufacturer told me to connect ground to the shield only on one side (in order to avoid ground loops), here shield is gronded on the computer side. The documentation of my RS485 adapter identified the pins for 2-wire connection as +RX/TX (B) and -RX/TX (A) and I confectioned the cable for exactly these two pins. In my case the rate was only 19.2 kBit/s and the cable length was less than 10 meters, and therefore resistors in the path were not necessary.


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## msplsh (Jun 24, 2022)

jbodenmann said:


> unbalanced interface whereas your adapter expects a balanced interface


No, the device is half-duplex.  The two wire connection is "balanced" in the way XLR cables are and the G is ground.


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## msplsh (Jun 24, 2022)

obsigna said:


> This is not a definitive answer.


But it's good enough to corroborate what I have!


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