PLC Interfacing with PIC

[rhino]

If you're looking for an "off the shelf" solution (which most of us are not) look at opto22 modules or similar.

[tenaja]

I've used quite a few Opto22 modules. (Opto's plant is just across the street from me.) They work well, but check the spec sheet on timing, if you need high speed.

On my last project, I used Grayhill's Opto22 pin-compatible modules, and saved quite a bit of money. They worked just as well as Opto's.

[skian85]

Thank you so much for all your advise. I will try them. Maybe i can use zener diodes to replace LM7805. I appreciate your effort and time.

[George]

Just use a resistor! The input pin is limited to vcc so a resistor works quite well and is very cheap and simple - if the output of your PLC is left floating you may have to tie it down with another resistor. I've used a resistor straight into a PIC from 240Vac for sensing the zero crossing to synchronize firing a TRIAC (I think I used some where in the region of 200k). It seems so wrong I know, but it really works well and is simple.

[George]

Just make sure with yr resistor you don't exceed 20mA - try keep it down to 1-2mA