Experimental Physics and
Industrial Control System

Mark, that did fix the problem - changing nbits in INP asynMask from 16 (for unipolar) to -16 (for bipolar). Thanks so much!  -- Jon

PS - so now small negative input voltages result in small negative integer RVAL,
with Vin -0.005 V:
epics> dbgf Iso:GantryCouch:Collimator:Actual.RVAL
DBR_LONG:           -8        0xfffffff8

On Mon, 14 Dec 2015, Mark Rivers wrote:

I believe I may have found your problem.
 ...
 field(DTYP, "asynInt32")
 field(INP, "@asynMask(CollimCouch, $(OFFSET), 16, 1000)MODBUS_DATA")

So you are using asynInt32 device support with the asynMask link syntax.

This is the documentation from for asynInt32 and asynMask from http://cars9.uchicago.edu/software/epics/modbusDoc.html ...

field(INP,"@asynMask(portName,offset,nbits,timeout)drvUser")
....
nbits>0 for a unipolar device. For example, nbits=12 means unipolar 12-bit device, with a range of 0 to 4095. nbits<0 for a bipolar device. For example, nbits=-12 means bipolar 12-bit device, with a range of -2048 to 2047)
...
So you have set nbits=16.  But the documentation explains the nbits>0 is for unipolar devices.  Yours is not a unipolar device, it is a bipolar device, so you should be setting nbits=-16.

I believe that will fix your problem.

References:

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

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