Micro 850 to Inductive Automation Ignition via Modbus TCP

The problem is that the Micro 800 tags are mapped to Modbus "Output Coil" addresses that begin with the prefix digit "0", rather than Modbus "Holding Registers" that begin with the prefix digit "4".

The test driver (and presumably any other HMI drivers) is configured to use Modbus Function Code 0x03, which is "Read Holding Registers".

The Micro 800 doesn't have any Holding Registers defined, so it replies with a "invalid address" error code.

First, try using Modbus Function Code 0x01 in the Simply Modbus test tool, which will read Coil type data.

Then either configure your HMI to to read "Coil" addresses, or configure the Micro 800 to use "Holding Register" addresses.

EDITS:

Yes, I meant Modbus/TCP on Port 502, not 503.

I wrote my post before seeing the error code when attempting to read Coils, and will take a look at that next.

Simply Modbus is a runner-up in bad interface design to RealTerm.

"First Coil" =1 and "minus offset" = 1 results in an actual Coil offset value of zero.

00 2E 00 00 00 06 01 01 00 00 00 06

I believe that the CCW software identifies the first Coil, with offset = 0, as "000001", so that ought to work.

Have you tried a single frame that will read all twelve of the defined Coils, instead of just six of them ?

The Micro 800 really is rejecting this request from the Modbus TCP client, I just can't figure out why.

When you add MQTT Gateway to the network everything become "Tag-Based"!

I respectfully submit that nothing is as simple as Modbus, and that simple solutions should be tried before complex ones.

It's fine that you're enthusiastic about MQTT and edges and gateways and Internets made up of Things. But this is a Modbus addressing mistake.

Ken - Point well taken!