In Control

I just got asked about how many devices can be supported on a ModBus link. There is much confusion around such questions, so here's a quick summary of the facts.

ModBus is a protocol. It defines messages, responses and data content. It is the language. It says nothing about the communications interface (lips, tongue, vocal chords). Other protocols include TCP, X-modem and Morse code. (If you are knowledgeable enough to debate me on the latter, you probably don't need to be reading this :-)

RS232 is a communications interface. The RS232 standard defines some electrical voltage and current levels. It says nothing about the language (including nothing about the number of bits, parity etc., which strictly speaking are the character profile, something else again. Phonemes?)

RS485 is another electrical interface. So are RS422, Ethernet, current loop, V35 etc.

So maybe now you can see why questions like "do you have a ModBus interface" or "do you have RS485 protocol" are not the right question to ask.

So how many widgets can I connect?

With apologies to Damon Runyon

I'm sitting at my desk late one afternoon, thinking I could get lucky and be out of here by 5 o'clock, when the phone rings. I pick up the phone and I say as follows: "Hello, SPLat support line".

On the 'phone this voice says: "Is 'dat 'de SPLat support line?".

Now, where I come from if someone answers the phone with "Mindy's Restaurant" I know I've gotten through to Mindy's. In fact, if I dial the number for Mindy's I'm going to give very long odds indeed against the phone being answered "White House Washington". So I decide to ignore a question being answered with a question, and I say: "How may I help you?"