Early ROM revisions on the Webster MSCP controllers were too slow for the KA650 and would often loose interrupts (amoungst other things). Signals such as NPR and BR grants were polled from a pseudo microcode loop which took too long. The RS232 interface doesn't use standard receivers. Check for a 5 volt zener on the input of an 74LS240 (from the 10 pin i/o connector) and add a schottky in parallel (a 1N5818 would do) if one isn't there