Modbus TCP Question

Yay it works! Modbus TCP is fun because everyone implements it differently but the protocol is so simple.
If anyone knows the modbus creator, please ask them if they regret defining address 0x00 as Address 1.

Now that I know it is working I have to figure out the rest of my headaches:
- Reading this through either my Red Lion HMI or through the CompactLogix.

Click to expand...

If using the Red Lion you might not need the offset. I was set up using the PLC to read the data and for that I had to subtract 1, but for the Red Lion I didn't. If I had to choose again I would go straight for the Red Lion, it made Modbus very easy. Well very easy once I got my head round 'Gateway Blocks'.

It seems to be up to the slave whether it wants a device address (unit number). Sometimes 0, 1, or 255 are the correct answer. Why they have it still in TCP? Who knows!?

You can use modbus TCP as a TCP to RTU gateway. So if you had address 34, it would send a message on the 485 bus to node 34. If I were implementing a TCP slave stack I would accept any address. If I was implementing a TCP master stack I would allow to change this number. But not every stack maker does it this way.