RE: ai RVAL via asyn + modbus: getting unsigned integer

[jon@u.washington.edu]

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