Interfacing the Standard Parallel Port http://www.senet.com.au/~cpeacock
Interfacing the Standard Parallel Port Page 5
Address Notes:
3BCh - 3BFh Used for Parallel Ports which were incorporated in to
Video Cards and now, commonly an option for Ports
controlled by BIOS. - Doesn't support ECP addresses.
378h - 37Fh Usual Address For LPT 1
278h - 27Fh Usual Address For LPT 2
Table 2 Port Addresses
When the computer is first turned on, BIOS (Basic Input/Output System) will determine the
number of ports you have and assign device labels LPT1, LPT2 & LPT3 to them. BIOS first looks at
address 3BCh. If a Parallel Port is found here, it is assigned as LPT1, then it searches at location 378h.
If a Parallel card is found there, it is assigned the next free device label. This would be LPT1 if a card
wasn't found at 3BCh or LPT2 if a card was found at 3BCh. The last port of call, is 278h and follows
the same procedure than the other two ports. Therefore it is possible to have a LPT2 which is at 378h
and not at the expected address 278h.
What can make this even confusing, is that some manufacturers of Parallel Port Cards, have
jumpers which allow you to set your Port to LPT1, LPT2, LPT3. Now what address is LPT1? - On the
majority of cards LPT1 is 378h, and LPT2, 278h, but some will use 3BCh as LPT1, 378h as LPT1 and
278h as LPT2. Life wasn’t meant to be easy.
The assigned devices LPT1, LPT2 & LPT3 should not be a worry to people wishing to
interface devices to their PC's. Most of the time the base address is used to interface the port rather
than LPT1 etc. However should you want to find the address of LPT1 or any of the Line PrinTer
Devices, you can use a lookup table provided by BIOS. When BIOS assigns addresses to your printer
devices, it stores the address at specific locations in memory, so we can find them.
Start Address Function
0000:0408 LPT1's Base Address
0000:040A LPT2's Base Address
0000:040C LPT3's Base Address
0000:040E LPT4's Base Address (Note 1)
Table 3 - LPT Addresses in the BIOS Data Area
Note 1 : Address 0000:040E in the BIOS Data Area may be used as the Extended Bios Data Area in
PS/2 and newer Bioses, and thus this field may be invalid.
(96 pagine)
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