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Great Hierophant
April 3rd, 2006, 06:14 PM
The IBM PC supports three parallel ports and four serial ports. But how many can you get with IBM options? For the PC, IBM put out a card called the Printer Adapter and another card called the Asychronous Communications Adapter. Each was a half-length card that supplied one physical parallel or serial port per card. IBM did not put out more advanced cards until the PS/2 line was released. However, the IBM BIOS could recognize up to three logical LPTs (with a Monochrome Display and Printer Adapter), each using the same IRQ and different I/O ports and four COM ports, with COM1/3 sharing IRQ4 and COM 2/4 sharing IRQ3 and all four COM ports using different I/O addresses.

The BIOS is only half the battle for these cards. Each parallel and async card's address decoding logic assumes that it is the first LPT or COM. I see the potential for bus conflicts on one hand and a true lack of individuality to the ports on the other hand. I have seen pictures of the true IBM cards and nowhere is there jumpers or dipswitches for changing the address of these cards. Does it require a hardware modification like the PCjr.'s parallel printer attachment?

modem7
May 29th, 2006, 04:16 PM
I have both these cards, and they are easily configurable.

Both cards have what I think were called jumper blocks (it's been a while). They sit in IC sockets. To change the configuration, you pull out the jumper block, rotate it 180 degrees then reinsert it.

The IBM asynch card has two: one at U15 and one at U5. On one alignment, U5 configures the card to port 03F8 using IRQ 4, and the other alignment configures the card to port 02F8 using IRQ 3. I can't remember what U15 is for.

I can't find my IBM Parallel card right now, but it uses the same configuration mechanism.

modem7
May 29th, 2006, 08:51 PM
I couldn't find my IBM parallel port card (did it even exist?), but I did locate another card. In the move to the AT, IBM combined a serial and parallel port onto the one card. It uses the same configuration mechanism I stated earlier. It has two 'jumper blocks' (or whatever they're named) labled J1 and J2.

modem7
June 1st, 2006, 01:20 AM
Okay. Turns out that what I thought were called 'jumper blocks' were in fact referred to by IBM as 'shunt modules'.