It shouldn't be that hard but one has to ask if it's worth the hassle. There is so little info on the web about your boards that they were either not very popular or went out of production a long time ago. NXP apparently still sell the 8051 variant used in your boards. From what you have posted all you need is to add a standard serial port to your Pi (you can buy USB adaptor to do this) and then you'll need to write the code to talk to the boards. If you have access to complete documentation of the serial prototocl they use it shouldn't be too hard but it may take a while.
MK
