This article explains how to identify and fix the most common causes of RS-485 parity errors, frame errors, and “Node is offline” messages on Anybus Communicators with serial interfaces.
APPLICABLE PRODUCTS
Anybus Communicator (all RS-485 serial variants)
PRE-REQUISITES
A serial ABC Anybus Communicator
IN THIS ARTICLE
Check RS-485 Cabling & A/B Polarity
Verify RS485+ (line B) and RS485− (line A) wiring
Swap lines A/B if communication fails
Confirm that the cable shield has a good connection to FE
Steps:
- Make sure RS485+ on the Anybus is wired to RS485+ on the connected device, and RS485− to RS485−.
Incorrect polarity often causes: Parity errors, Frame errors, No response (device appears offline) - If wiring looks correct, but communication still fails: Swap RS485+ and RS485−.
- The cable shield must be connected to Functional Earth (FE) on the Anybus device. Do not connect FE to signal ground. The signal ground of the Anybus device must be connected to signal ground of the serial device if needed.
Verify Termination & Biasing
Steps:
The RS485 connection must be terminated at both physical ends.
Correct termination:
120 Ω resistor at the first device
120 Ω resistor at the last device
Additionally, in environments where EMC radiation can be expected, it is advisable to use an active termination on one side of the RS485 bus with biasing resistors. This can conveniently be done on the Anybus Communicator side, since signal ground and +5 V are readily available.
Correct active termination:
In addition to the above 120 Ω resistors, connect two 470 Ω resistors as depicted below.
Please see the image for reference: 
ADDITIONAL INFO
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