Experimental Physics and
Industrial Control System

[email protected] · Thu, 3 Dec 2015 10:16:48 -0800 (PST)

On Thu, 3 Dec 2015, Mark Rivers wrote:

Hi Jon,

http://cars9.uchicago.edu/software/epics/modbusDoc.html

Supported Modbus data types
modbusDataType value    drvUser field   Description
0       UINT16  Unsigned 16-bit binary integers
...
4       INT16   16-bit signed (2's complement) integers. This data type extends the sign bit when converting to epicsInt32.
...
So modbusDataType=0 is UINT16, unsigned 16-bit integer.  That explains your problem, you should be using modbusDataType=4 (INT16) and your command line should be:

drvModbusAsynConfigure("CollimCouch", "AcromagCollimCouch",0,4,0,9,4,100,"AcromagXT1221")

That must be it. The referenced documentation is for Modbus release 2-7 and

we are still running 2-2, which only supports data type codes 0 - 3. When we try

code 4 in 2-2, RVAL is always 0. The 2-2 doc says:

Type 0 is used for binary numbers in twos-complement format. The data can either be signed or unsigned.

But, as we found, it isn't very helpful for signed integers.

Thanks for clearing this up.

Jon Jacky

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System