Hi,
Schmitt trigger is correct.
In our applications we don't have a display, all data are transferred with profibus.
And while we often use Atmel AVR type microcontrollers, I don't recommend them for your application because of limited processing power, ressources like memory and limited interface capabilities. Maybe an ARM based microcontroller is more suitable, or a DSP.
Our microcontrollers are supported by CPLDs or FPGAs. I personally like the idea of doing thos (safety) critical tasks in an FPGA rather than by software. I think a software is more likely to fail than an FPGA program. But indeed I never saw our microcontrollers to fail this way. A good PCB layout, shielding, filters on all IOs are mandatory.
We prefer the data communication via POF (plastic optical fiber) because of it's immunity against electromagnetical interference.
(We control up to 2500V and up to 6000A RMS, causing a lot of interference)
Klaus