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?

We have become aware that some customers are running SPLat controllers with backlit LCDs 24/7. That can result in eventual loss of backlight intensity, as the lifetime of small white LEDs is around 3,000 hours (an inherent limitation of current technology). We have therefore put on our thinking caps and engineered some ways of extending the backlight lifetime by dimming or switching off the backlight when it's not being viewed.