Experimental Physics and
Industrial Control System

Mark Rivers <[email protected]> · Thu, 3 Dec 2015 18:37:37 +0000

Hi Jon,

I didn't realize you were running an old version of modbus.

In R2-2 this is from the readInt32 function:

            switch(pPlc->modbusFunction) {
                case MODBUS_READ_COILS:
                case MODBUS_READ_DISCRETE_INPUTS:
                case MODBUS_READ_HOLDING_REGISTERS:
                case MODBUS_READ_INPUT_REGISTERS:
                    *value = pPlc->data[offset];
                     break;

pPlc->data is unsigned short, so it will be copied to *value without casting to signed.

In R2-4 and later this is from readInt32:

            switch(pPlc->modbusFunction) {
...
                case MODBUS_READ_HOLDING_REGISTERS:
                case MODBUS_READ_INPUT_REGISTERS:
                case MODBUS_READ_INPUT_REGISTERS_F23:
                    status = readPlcInt(pPlc, offset, value);
                    if (status != asynSuccess) return status;
                    break;

This is from  readPlcInt:

    switch (dataType) {
        case dataTypeUInt16:
            result = value;
            break;
...        
        case dataTypeInt16:
            result = (epicsInt16)value;
            break;

So for data type 4 (dataTypeInt16) it casts the value to epicsInt16 (signed) before copying to the epicsInt32 value.

If you want to correctly handle the data from your device you need to upgrade to modbus R2-4 or later.

Mark

________________________________________
From: [email protected] [[email protected]]
Sent: Thursday, December 03, 2015 12:16 PM
To: Mark Rivers
Cc: [email protected]
Subject: RE: ai RVAL via asyn + modbus: getting unsigned integer

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