How and when are Modbus points polled

Issue

How and when are Modbus points polled

Product Line

SmartStruxure Solution

Environment

AS ES Modbus TCP RTU

Cause

Need to understand the Modbus poll mechanism

Resolution

See StruxureWare Technical Reference Guide, Modbus Deep Technical Reference chapter

The Modbus poll manager manages all polling of the points/data and devices.
The poll list contains all the current subscriptions (eg. data being referenced within Programs, Listviews, graphics, logging etc)
So the programs and graphics are isolated from the Modbus Polling.

NOTE
1. For each network, the poll manager operates with a poll list and a poll task.
2. If a Modbus Object value is only required for logging purposes, then the  "Requested poll interval" is set to a high number (typically 2,147,483,647 ms) This means the object values is not currently being polled.
When the time arrives for the log value to be collected, the "Requested poll interval" is set to 1000ms, until the value is collected, at which point it reverts to this higher number.