At 10:14 AM 12/6/2005 -0800, you wrote:
>A couple of years ago did a project for a client which included 
>implementing a Modbus slave (RTU) on a PIC18F252. At the time I remember 
>it was kind of strange that although Modbus is de facto standard, I 
>couldn't find any example code for a slave. (There was lots of code for a 
>Modbus master.) A few things I remember off the top of my head:
>
>- Data is big endian
>- Data is word based (two bytes); there's nothing in the spec about 
>floating-point values or strings, so I had to use trial and error.
>- The freely available documentation isn't very thorough. There were no 
>examples of byte-for-byte messages including CRC. (This may have improved 
>since then.) I had to download a free PC-based Modbus master program, and 
>look at the data with a binary terminal program. My problem was that my 
>CRC bytes were swapped. Did I mention that modbus is big endian?
>- The documentation includes lots of controller (mostly PLC) specific 
>stuff that doesn't apply for most implementations, so I didn't implement 
>some of the messages.
>- Modbus messages don't include a start-of-message, length field, or 
>end-of-message.
>
>Those are just a few of the things I remember at the moment, but I do 
>remember it was a pain. Anyone else out there who has implemented a Modbus 
>slave have similar experiences?

Did it (ASCII + RTU) with an 8051 variant a few years back. It's a pretty
simple protocol, but the way the RTU framing works I think I had to dedicate
an on-chip timer to it to meet the specs completely without using (ugh) delay
loops.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


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