Parallel Port - Implementation On IBM Personal Computers - Port Addresses

Port Addresses

Traditionally IBM PC systems have allocated their first three parallel ports according to the configuration in the table below (if all three printer ports exist).

LPT1 IRQ 7 0x3BC 0x3BF
LPT2 IRQ 7 0x378 0x37F
LPT3 IRQ 5 0x278 0x27F

If there is an unused LPTx slot, the port addresses of the others are moved up. (For example, if a port at 0x3BC does not exist, the port at 0x378 will then become LPT1.) The base address 0x3BC is typically supported by printer ports on MDA and Hercules display adapters, whereas printer ports provided by the mainboard chipset or add-on cards rarely allow to be configured to this base address. Therefore, in absence of a monochrome display adapter, a common assignment for LPT1 today is 0x378, even though the default is still 0x3BC (and would be selected by the BIOS if it detects a printer port at this address). The IRQ lines are typically configurable in the hardware as well. Assigning the same interrupt to more than one printer port should be avoided and will typically cause one of the corresponding ports to work in polled mode only. The port addresses assigned to each LPTx slot can be determined by reading the BIOS Data Area (BDA) at 0000:0408.

Bit to pin mapping for the Standard Parallel Port (SPP):

Bit: 7 6 5 4 3 2 1 0
Base (Data port) Pin: 9 8 7 6 5 4 3 2
Base+1 (Status port) Pin: ~11 10 12 13 15
Base+2 (Control port) Pin: ~17 16 ~14 ~1

~ indicates a hardware inversion of the bit.

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