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Modbus write command setup and timing

Modbus write command setup and timing

Issue

Modbus write command setup and timing

Product Line

EcoStruxure Building Operation

Environment

  • Building Operation Automation Server (AS, AS-P, AS-B)
  • Building Operation Enterprise Server (ES)

Cause

The Modbus value keeps being sent to a Modbus register (when write code 06 or 16 used)

Resolution

Control of the write timing is via the binding to the Modbus point within the Modbus network.

  • If the Communication type is set to "Periodic" then the value will be sent based on the Period Communication Interval.
  • If the Communication Type is set to "Change of Value" then the value will only be written when the value changes by this amount.

The Re-transmit Interval should also be considered, as the register will also be written to based on this timing value, regardless as to whether it has changed or not. This value may need to be increased if the value should not be re transmitted apart from when the value changes.

Up to v1.3

v1.4 and later

Some Modbus devices get over these sort of issues by having a mode that can be set so that re-transmission of the same value is ignored, as the example below

NOTE
Block (Batch, Multiple) Writes are not supported (correct as 2020 01 01 and up to v3.1) Although they are used to write to 32, 48 & 64 bit Register types (so "Write Multiple Registers" packets will be seen when writing to 32, 48 or 64 bit registers) 

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