Four processors would make it a 3540... How much stuff is on the QBus adapter board? There were two versions: FTAM supports only the TK70 controller (is you TK70 attached to QBus or to SCSI?) and FQAM supports any random QBus module. FTAM is a simple module containing only a few chips; FBIC (interface to the backplane) and CQBIC (CVAX -> QBus adapter) being the only really big ones. I believe there was also a clock generator chip, but not much else. FQAM is chuck full o' goodness. It his a prom-based microcode machine to run the module and lots and lots of PALs. The connector which goes out the side of the backplane is SCSI. Although DEC used the DSSI chip on this module, they only supported SCSI; the DSSI chip is capable of doing both. The connector which comes out the front of the backplane goes to a distribution panel which provides: - Four serial ports (console/printer, modem, mouse, keyboard) - Ethernet (AUI and BNC) - Diagnostic code display (one 7-segment LED) Unfortunately, I don't know the pinout of the cable; I don't have a printset for the 3520. Three boards is correct. For extra fun, it has its own embedded MicroVAX II... The terminal plugs into the distribution panel which plugs into the I/O module.