Interfacing Microcontrollers With Industrial Devices

[rantonides]

I would love to see a video about going from Ttl signals to 24v for industrial interfaces.  I bet there's a bunch of industrial guys that would like to move from a PLC to Microcontrollers.  Love to see ideas for using sinking or sourcing 24v inputs and outputs and convert to protected controller signals.

Rob

[Jon Chandler]

EDN had a great Design Tip about this about a year ago.  Outputs use ULN2803s for compatibility with 24v or even greater systems.  Inputs use MC1489 buffers to convert high voltage levels to 5 volts.  It looks like a cheap, reliable method.

[rantonides]

That's a nice inexpensive solution.  Unfortunately, on this job, my client likes PNP sourcing signals.  They find it easier to troubleshoot when you can just measure a signal to ground.  If you get 24V, it's on.  If it's 0V, it's off.  The solutions I've found so far use MOSFET IC's (high side switches) which bump up the price to $3-4 per channel.  There must be a less expensive way to have protected sourcing inputs and outputs that are friendly to switching coils.

[Psi]

Have you looked for driver / PNP transistors array in IC packaging (4+ per chip etc).
Might be cheaper than individual components


Found this one (A2982SLWTR-T) which is a similar chip to what was used above but it's got 8x sourcing outputs instead of syncing
- max 500mA per channel
- up to 50v
- TTL,Schottky TTL, DTL, 5V CMOS inputs
- DIP/SOIC packaging
- Internally protected from transients

It even says their intended application is driving relays/solenoids/steppers/servos etc. 

http://search.digikey.com/nz/en/products/A2982SLWTR-T/620-1127-1-ND/1090370
Digikey have them in stock (US$2.84)

[Rufus]

That's a nice inexpensive solution.  Unfortunately, on this job, my client likes PNP sourcing signals.  They find it easier to troubleshoot when you can just measure a signal to ground.

PNP type +ve logic signals are almost universal in 24v control systems because unintentional short circuits are expected and the most likely short circuit (to ground) produces a safer off rather than on condition.

Expected short circuits also rule out crappy darlington driver chips (NPN or PNP) which would be destroyed instantly and their high saturation voltage makes them useless for high current applications.

I already pointed you at the ITS711, digikey have stock of its slightly more capable brother the ITS724 which provides 4 x 70mOhm 3.3A drivers for $4 10 off. There isn't a cheaper way of doing it unless you significantly reduce the output current requirement.

Inputs are easier and not with MC1489s which take more power than the processor in that example, have 30v abs max input rating, and would cook with 4 24v signals on them.

[Rudolfo]

You may take a look to this article on interfacing microcontroller to the industrial world:
http://www.embedded-know-how.com/chips-a-components/articles-a-white-papers/article/chips-components/46-articles-white-papers/353-interfacing-microcontrollers-to-the-24v-industrial-world.html