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Subject: RE: reading negative 16-bit numbers over Asyn modbus
From: Mark Rivers <rivers@cars.uchicago.edu>
To: "'matthew.pearson@diamond.ac.uk'" <matthew.pearson@diamond.ac.uk>, "tech-talk@aps.anl.gov" <tech-talk@aps.anl.gov>
Date: Thu, 5 Apr 2012 18:00:03 +0000
Hi Matt,

Support for Modbus data types 4-10 were only added in Modbus R2-4, which is in beta release.  Previous versions do not support signed 16-bit integers, only unsigned.

You can get the 2-4beta1 version at:

http://cars.uchicago.edu/software/pub/modbusR2-4beta1.tgz

I'd like to release it soon, so if you could test if it works for your application that would be great.

Mark


-----Original Message-----
From: tech-talk-bounces@aps.anl.gov [mailto:tech-talk-bounces@aps.anl.gov] On Behalf Of matthew.pearson@diamond.ac.uk
Sent: Thursday, April 05, 2012 12:51 PM
To: tech-talk@aps.anl.gov
Subject: reading negative 16-bit numbers over Asyn modbus

Hi,

I have a Eurotherm 3217 controller that I'm talking to using modbus RTU over RS485 (via a Moxa terminal server). It seems to work fine, but I'm having trouble reading a negative number.

The Asyn support is configured to use modbus function code 3, like this: (from the eurotherm2k support module eurothermModbus.c):

mkname(str, strmaxlen, asynPortName, modbusAddress, loopNumber, 0, "PV");
asynRet = drvModbusAsynConfigure(str, asynPortName, modbusAddress,
		MODBUS_READW, loopStart+PV_OFFSET, 3,
		0, EUROTHERM_POLL_MS, "");

Where:
loop number is 1.
modbus addr is 1.
MODBUS_READW is 3 (this is the function code, so it's reading 16-bit words).
loopStart+PV_OFFSET is 1+0, so 1.
3 is the modbus length, which means 3*16-bit.
The 0 data type is Unsigned 16-bit binary integers.

This works fine for positive numbers, but negative fails. I tried changing the modbus data type to 4, which should mean INT16 2's complement, but it seems to fail at some point and just return zero in the record.

If I turn on asyn trace, the negative word I am reading back is "fd 1c". The actual reading I expect is about -750. So this looks to be correct if assuming 2's complement encoding. 

Anybody successfully read negative integers over modbus?

I'm using modbus 2-2, and asyn 4-14.

Update... looking at drvModbusAsyn.c, I see that convertFromBinary() only implements 3 types of type conversion (modbus data types 1, 2 and 3 in the list in the documentation?)

Cheers,
Matthew



Dr Matthew Pearson
Senior Software Systems Engineer
Diamond Light Source
Chilton, Didcot
OX11 0DE
UK

MX Controls Team Leader
Beamline Controls - I19, I24, B24




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