Confusion with RS485 data Tx+, Tx-, Rx+, Rx-, A, B, Y, Z

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udhay_cit

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I'm confused with the RS485 & RS422 Terminal notations. As per the datasheet of MAX485 pin description,
A = Noninverting Receiver Input and Noninverting Driver Output (Rx+ & Tx+)
B = Inverting Receiver Input and Inverting Driver Output (Rx- & Tx-)
Y = Noninverting Driver Output (Tx+)
Z = Inverting Driver Output (Tx-)

But as per the internet search most of them market as below,
A = Tx-
B = Tx+
https://www.bb-elec.com/Learning-Ce...erminals-are-used-when-connecting-the-48.aspx

which one is correct & which one i can follow. Please anyone explain me in detail.

Regards
Udhay
 

Hi,

You need to cennect:
Tx+ --> Rx+
Tx- --> Rx-
In both directions.

RS422 usually is full duplex, it uses two pairs for bidirectional transmission, point to point.
RS485 can be full duplex, half duplex and has bus capabilities. One or two pairs are needed forbidirectional transmission.

Example: Profibus is RS485, half duplex bus with only one pair for many devices.

Klaus
 

Hi Klaus,
Thanks for the reply. Yes i can understand the operation of RS482 & RS422, but my question is i need to replace the A, B, Y, Z of the MAX491 pin name with Tx+, Tx-, Rx+, Rx-. Is it A= (Rx+) or (Rx-)?

As per the datasheet description 'A' is Rx+ (Noninverting Receiver Input) (in RS422 4 wire mode). But as per the internet search 'A' is Rx-. Which is true?
 

EIA/TIA-485 defines signal levels for balanced data transmission but doesn't assign logic levels to A-B differential voltage.

An explicite assignment is given in specific interface standards like PROFIBUS or MODBUS. Unfortunately the usage of "A" and "B" doesn't correspond to the usage in RS-485 driver datasheets. But strictly spoken, the driver datasheets don't assign logic levels, respectively only suggest an assignment based on a specific usage of the logic input and output. If you connect a RS-485 driver to a standard logic level UART, you get line A with idle state "H" and B with idle state "L", which is opposite to the definition in MODBUS and PROFIBUS.

Which interface standard are you referring to?
 

Hi FvM,
I'm using my own USB to serial converter & marked the terminal as A,B. Now i need to connect this for MODBUS communication. I will always mark the MODBUS terminal as A,B. The RS485 is a bidirectional communication so there is no meaning to use Tx+ or Tx- alone & the best notation is A, B. But unfortunately here i have to use different identification except A, B (example: Tx+, Tx- or D+, D-).

I have attached the wiring diagram as attachment. Please correct if I'm wrong.
 

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