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PLC cannot talk to an Altistart 22 (ATS22) via Modbus read multiple registers

Published date: 17 July 2019

Issue
PLC cannot talk to an Altistart 22 (ATS22) via Modbus read multiple registers

Product Line
Altivar Variable Speed Drives and Soft Starters

Environment
PLC Programming

Cause
The ATS22 does not support Modbus Function Code 23 (Read Multiple Registers). Function Code 23 is the most commonly used Modbus packet requests for Schneider Electric PLCs so it is likely that you will just need to configure your PLC to use the supported function codes for the ATS22.

Resolution
Use the Modbus Function Codes 3 and 6 instead of 23.

See the Document: "ATS22 User Manual" (Reference BBV51330)
https://www.schneider-electric.com/en/download/document/BBV51330/
Starting at page 68, it describes the supported Modbus Function codes to read and write data:
  • Read Holding registers (Function Code 3)
  • Write one output word (Function Code 6)
  • Write multiple registers (Function Code 16)
  • Read device Identification (Function Code 43)

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