Good news! I've tracked down one of the programming manuals :)

http://www.usedplcs.co.uk/usedplcs/m...s%20Manual.pdf

Bad news! Even if you can get past the instruction set it looks to be a bit of a dog :(

Following Gary's comment I though it may be possible to write your own Modbus protocol handler, the ASCI version is quite simple. Looking at the programmers manual I can see no way of accessing the COM port directly, I can see a way of possibly sending messages, I cannot see a way of receiving them.

The Satcon05 range included a fancy basic module which may provide more flexibility, which CPU modules do you have?

Another possibility would be to link the parallel port inputs and outputs directly to one of the digital I/O modules. You could perhaps interpret the intentions of Mach3 or EMC and perform some additional processing before passing on the drive or returning the input but I don't know what this would achieve.

One last worry, the maximum speed of the 'fast' inputs appears to be 2kHz and the few scan times mentioned are in milliseconds, I suspect these old units are too slow to do anything meaningful?

To end on some good news, the controllers do have built in PID support. Perhaps useful for spindle speed control but again old hardware that may be better suited to controlling the temperature of your home-brew.