B&B Electronics SPP-100 - Manual - Manual (Page 58)

Appendix A: Parallel Port Terminology
A.1 Explanation of parallel port terminology
Starting with the original IBM PC, IBM defined the standard parallel
printer port. This port uses a female DB-25S connector on the computer,
and a special male DB-25P to Centronics male 36 pin "Printer cable" is
used to connect to standard Centronics parallel printers.
IBM defined three standard port base addresses (in 80x86 IO address
space). The printer adapter could use base address 0x378, or later 0x278,
while the monochrome display and printer adapter used base address
0x3BC. The SPP-100 and other EPP compatible parallel ports should not
use base address 0x3BC to avoid I/O conflicts.
A PC’s BIOS defines RAM space for 4 parallel printer port base addresses,
stored as 4 16 bit words starting at main memory address 0x408. During
bootup, the BIOS will check for parallel printer ports at base addresses
0x3BC, 0x378, and 0x278, in order, and store the base addresses of any
that are found in consecutive locations in this table.
DOS maps the BIOS table entries as LPTn devices. The first entry in the
BIOS table becomes LPT1, the second entry LPT2, and the third entry
LPT3 (if there are that many). Other operating systems and some DOS
based software support parallel ports located at non-standard addresses.
Nonstandard parallel ports may or may not be mapped as LPTn devices,
depending on the software or operating system.
The primary printer adapter (base 0x378) and the monochrome display
and printer adapter (base 0x3BC) are allocated hardware interrupt 7.
Secondary printer adapters (base 0x278) are supposed to use IRQ 5 for
their interrupt. These IRQs are commonly "stolen" for other usage since
most (nearly all) DOS software does not use the printer port IRQ for
driving printers. Other operating systems may use it for the printer port,
as may some parallel port transfer programs.
v If a parallel port device is configured at an LPT port, the I/O base
address assigned to the parallel port device is added to the BIOS
equipment list located at system memory location 0040:0008.
A-1 Quatech Inc.