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?
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?