Modbus write command setup and timing

Issue

Modbus write command setup and timing

Product Line

SmartStruxure Solution

Environment

Modbus AS 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 Retransmit 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 retransmitted 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 retransmission of the same value is ignored, as the example below

NOTE
Block (Batch, Multiple) Writes are not supported (correct as 2018/8/06 and up to v2.0.1) Athough they are used to right to 32, 48 & 64 bit Register types