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Modbus RS-485 limitation on a 511, 527, 701, 711, 721, 731 and 913

Issue

Modbus values are not updating as excepted.

Modbus points are not working reliably

Environment

Xenta controllers 511, 527, 701, 711, 721, 731 and 913

Cause

The amount of modbus points being polled is excessive for the controller.

Resolution

The product data sheets indicates no limit to the modbus points but only 31 modbus devices over RS-485.

With a practical install, a limit of 300-400 Modbus points per Xenta server  would be applicable. If you exceed this level, communications become slower than acceptable.

Network performance can be affected when a large number of connection objects are in use ( direct transfers between Modbus and other protocols). There is no documented limitation, but the recommendation is not to exceed 300 connection objects per Xenta Server.

Connection polling period should also be considered with a large amount of connection objects

See Lessons Learned Article #6797 that discusses this topic.

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