When should the Modbus RTU Annex points be used?

Issue

What is the best way to update Modbus RTU points through the X Driver?

Product Line

Andover Continuum

Environment

Continuum Modbus RTU XDriver

Cause

Requesting values directly will cause additional polling to occur and may conflict with the Plain English polling program.

Resolution

Polling of the X Driver points should be performed by a single Plain English program (or sequenced programs) that ensure only 2 requests are made per second (See Lessons Learned #7680).

Whenever programs, graphics, alarms, list views etc are going to be used to read the X Driver values then annex points should be used.

An Annex point is a numeric point (not an X Driver point) that will get automatically updated by the X Driver if its name is put into Parameter 6 of the X Driver point.

Annex points are required for block reads, but even if you are doing an individual value read, this is the most efficient way of mapping the X Driver point value into another point that can be used by the rest of the system.

Another advantage of using annex points is that if an invalid or bad value is received by the X Driver, it does not update the Annex point even though sometimes the X Driver point itself can go to a NotSet or a 0 value until the next valid read is received. The annex point always holds the last known good value.

Also see Lessons Learned #5541 for an example application for polling Schneider Electric power meters.