Here's my situation.
I am reading a 16 bit integer holding register on my serial modbus RTU (by the way, it works fine with Mango). 
On OpenAPC, a value of 0 reads 0.  That's ok. 
A value of 1 at the controller reads 256 on OpenAPC (2 => 512, 3 => 768, 4 => 1024, etc.) 
The obvious logical assumption is that the byte order is reversed. 
But when I UNCHECK the invert byte order on the MODBUS definition under the "Basic" tab (originally checked), I always read a value of  65521 or xFFF1 on all the mapped registers regardless of their value.  There's no question that there's a problem here.
I have no idea how to modify the source code / recompile - just thought that you'd guys would like to know that the "Invert byte order" option does not work (unchecked mode) - seems like a simple to fix to make a simple modbus RTU functional.
Thanks again.
From: Jim Hart <>
Sent: Thursday, January 10, 2013 5:23 AM
Subject: Re: Modbus


thank you for your interest in our products.

The MODBUS plug-ins already support 16 bit integer values. The width
of them depends - conform to the MODBUS specification - on the chosen
IO mode. This mode influences if the data are 8 bit int, 16 bit int or
32 bit float.

In case you need something additional that is outside of the scope of
the MODBUS specification you can add this easily, the sources are open
and available at

Alternatively you can let us do this modification as soon as we have
some time for it. In this case we'd need detailed information which IO
mode you're using, how many bytes have to be concatenated and which
byte order they have.

Kind regards


On 09/01/2013, Albert Serrero <> wrote:
> Hi, I just set up OpenAPC and was running a test project with some success.
> I have a low cost modbus controller connected to my Windows 7 PC via serial
> USB.  Most software platforms that support modbus allow some modification of
> the type of data that is expected back from the modbus device:  floating
> point vs 16 bit integer at a minimum.  My device only supports 16 bit
> integer as many low cost modbus devices do.
> Is there a way to specify or adjust for this with the Modbus control?
> Thanks in advance.
> ajs

Kind regards

Jim Hart

Follow us at Twitter:!/openapc
openapc mailing list