Modbus driver - support for reduced cap on transfer sizes

[BevanWeiss]

I was reminded of this issue that I've run into a couple of times with Geo SCADA Expert and certain Modbus devices.

The best example is when using a Rockwell ControlLogix with a 1756-EN2T (for example).  It's possible to write some custom logic to handle Modbus transactions.  However, there are data size restrictions on the raw socket comms, which means that full length ModbusTCP packets can't be used.

I've gotten around this in the past by deliberately keeping gaps in my Modbus register map, so that Geo SCADA Expert won't try to consolidate a whole set of adjacent reads into one long multiple register read (ditto for writes).

However, it would be nicer if the Modbus data map could remain 'perfect' and the limitation on transfer lengths could just be configured in Geo SCADA Expert (i.e. the ControlLogix Modbus implementation can only handle a maximum of 120 registers in one request).

Limitations from the ControlLogix ModbusTCP shown below

[sbeadle]

Thank you.

As you know the Simple Modbus driver lets you control the polling because you manually configure the scanners.

[BevanWeiss]

Thanks Steve,

I've always considered the 'Simple' drivers as best avoided.  My understanding was that there was intended to be a deprecation of the Simple drivers where an Advanced driver had been developed.

i.e. The Simple Modbus driver was eventually not going to be shipped with the Geo SCADA Expert product (and ditto for the Simple OPC driver).