Aha! Unless you are planning on using the Printer Interface card (which contains a serial and a parallel port), I wouldn't worry about the S100 area. If the 0v pin (pin of U18 is not zero volts, then it's not grounded properly. There are some mistakes in the circuit diagram around U18 (on my copy at least). Make sure that on the left side of the chip it shows pins 2,7,10,12,15,5 and the right side should show 3 D0, 6 D2, 11 D3, 13 D4, 14 D5, 4 D1. edit: I've sent you an email, and I removed the address from your post, to spare you from the spammers.