Hello,

In the earlier versions of the Modbus Application Protocol specification, the exception response for the MEI Transport function (code 43 or 0x2B) used to have the following format:

Function code (1 byte)
MEI Type (1 byte)
Exception code (1 byte)

But in the latest publicly available version (v1.1b) the MEI Type (second slot) was silently taken out of it, so that the format looks like this now:

Function code (1 byte)
Exception code (1 byte)

I mean, I actually embrace that change, since the former proposal of the exception response structure was somehow inconsistent with the rest of the specification. I'm just wondering why that change was not mentioned in the spec, and which format of the exception response the most slave devices out there use.

Thanks in advance for your comments.

Best regards,
Ivo Bauer

 

Anyone, please?

 

There seems to be close to zero information on this anywhere. I have been working on some Modbus software, but I was using the latest spec and I didn't notice this (although I haven't implemented this function either). In fact, I had to look at the two spec versions carefully before I saw what you were talking about.

The two spec versions are only 2-1/2 years apart, so it is difficult to guess if there are a lot of products which use the older one. The problem with that sort of judgement though is that if there is a product which uses the older version, it may be a product which is really important to support. You might need to support *both* behaviours.

I think you would have to talk to someone at Modbus.org to get the background on this change. It is possible that the first version was an error in the spec and was never intended (or implemented) to be that way. If you find out, I would be interested to know this as well.