ferrouss@tekgen.TEK.COM (Ferrous Steinka) (03/26/88)
So, you've upgraded to a multifunction card and you have an old IBM asynchronous communications adapter laying around that is just taking up space. Well, with a simple mod to the board, it can be made to respond to COM3 or COM4. Of course you will need to have appropriate software to take advantage of your new serial port. IBM MSDOS 3.3 knows about ports 3 and 4, MSDOS 3.1 doesn't and I'm not sure about 3.2 or 3.21. I tested ProComm and it DOES know about the secondary ports. The addresses of COM1 and COM2 are 3F8(h) and 2F8(h) respectively. The addresses for COM3 and COM4 are 3E8(h) and 2E8(h) respectively. The difference is address line A4, and all you have to do is invert it on the board prior to the decoding logic. This can be done by cutting pin 5 of U-2, a 74LS30, at the surface of the board and gently bending it up so that it may be soldered to. The pad to which pin 5 of U-2 was soldered is A4, which connects via the edge card connector to the system bus. Pin 5 of U-2 is an input to the decoding circuitry. I chose to use a 74LS04 as the inverter (piggy-backed onto U-3, also a 74LS04, to provide power and mechanical stability) . Connect pin 3 of the tacked on 74LS04 to A4 and pin 4 of the 74LS04 to pin 5 of U-2. CAUTION: There are unused buffer/inverters on the board but they are open collector type devices. Using these could lead to timing problems. For additional information, consult an IBM Technical Reference Guide for either the PC or the XT. Both have schematics of the asynchronous communications adapter.