Using old network router as a plc?

[D.L]

Hi, i got given a box of old cisco routers from 1997 era. I googled a search for hacking these old devices and came up with a search result yeilding a result of somebody installing a barebones linux and using it for robotic control, it is a basic computer for running simple routines, much the same as a plc only far cheaper.
Has anybody embarked on reformatting the firmware made good on conversion?

Ideally I'd use it to run a routine to convert hexadecimal to step and direction for running a homebrew 4th axis on a Mazak RS232 connection.

My tech pal says it's feasible, the WRT linux is available, I do lack raw coding experience, it may be a mind bender, anyone?