Modbus

We have an issue with Modbus over RS-485 where the controlled pump seems to be insisting on us padding each command with leading/trailing filler bits.

Anyone experienced with such err features?

I am looking at

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and nearby. Is there a more definitive document defining ModBus over RS-485?

Reply to
David Lesher
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tirsdag den 8. juni 2021 kl. 22.05.43 UTC+2 skrev David Lesher:

does the bus have fail safe bias resistors?

Reply to
Lasse Langwadt Christensen

The Modbus/RTU framing is a good candidate for the sickest way of defining packet boundaries.

In addition to the pull up/down resistors mentioned by Lasse, are you sure that the interframe timings are met?

A serial interface chip with internal FIFOs will quickly mask the timing of the octets on the line, so the software has very limited control.

Reply to
Tauno Voipio

That's ....encouraging.....

The bothersome client uses a ADM2486. I can't recall what out end is.

Reply to
David Lesher

About a quarter century ago, I had to code a Modbus/RTU master/slave unit with MC68332 and four 16C550 UART's.

I had to disable the internal FIFO's and harness a pair of 68332 timers per Modbus channel, to get the timings to match the specification.

The ready unit had enough processing power to handle the increased interrupt load, in addition of the other things to do. The unit did obey the spec, we tested it with the customer. In the same testing session, we noticed that there were no PC Modbus handlers performing to the timing specifications.

Reply to
Tauno Voipio

The 332 was a very nice chip for its day.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

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