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Jon's Imaginarium – Industrial 24 Volt Microcontroller Interface, from EDN


Well-Known Member
Interfacing a microcontroller to industrial systems, which often operate on 24 volt signals, can be a challenge. From EDN, Microcontroller Interfaces to 24V systems, provides wide-range bullet-proof I/O. It uses old-school MC1489 quad RS232 interface chips and will handle a wide input range of up to 30v. Unlike voltage divider inputs or optically-coupled signals, it handles a wide input range without any adjustments being required. Outputs are handled by ULN2803 peripheral driver chips.

Note: this has been posted previously in response to a forum question. I'm reposting it here so that it can easily be found in the future.
24v io edn.jpg


Well-Known Member
Most Helpful Member
I like your other posts, but that's one I have to dispute!

It would work, but it is a terrible idea in practice.
A 1489 line receiver has a minimum input threshold of only around a volt and with trivial current at that; they will generally work at TTL logic levels.

The noise immunity using those would be nonexistent in a real industrial environment.

A properly designed 24V input interface has a fairly high threshold voltage and with enough input load to require a definite current above some threshold before the input is seen as "high".

eg. A Siemens S7-1200 PLC input needs 15V & 2.5mA minimum to switch on, and switches off at around 5V & 1mA.

And for a suitable output device, look at an MIC5891 - that's an 8 x high side driver suitable for 24V systems.

Open collector or ground-switched outputs vary from being frowned on to being downright illegal in some contexts as wiring fault is more likely to activate a device rather than be detected or blow a fuse.

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