Disclaimer: *** NO WARRANTY AT ALL ***

But the info given is correct. I hope. Well, check it out yourself if
you don't believe me.
 
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Some short notes on the printer port of the PC
==============================================
 
Base address
------------
 
Normally, this table is valid.
 
   LPT1  378h or 3BCh
   LPT2  278h or 378h
   LPT3  278h
 
The base address can be read from the BIOS table (cut from Ralf Brown's
interrupt list):
 
Format of BIOS Data Segment at segment 40h:
Offset  Size    Description
 08h    WORD    Base I/O address of 1st parallel I/O port, zero if none
 0Ah    WORD    Base I/O address of 2nd parallel I/O port, zero if none
 0Ch    WORD    Base I/O address of 3rd parallel I/O port, zero if none
 0Eh    WORD    [non-PS] Base I/O address of 4th parallel I/O port, zero if none
                [PS] Segment of Extended BIOS Data Segment
                    Note: Above fields filled in turn by POST as it finds
                    parallel ports. POST never leaves gaps. DOS and BIOS
                    parallel device numbers may de redefined by re-assigning
                    these fields.
 
Registers
---------
 
  Base + 0   Data Register (r/w)
  Base + 1   Status Register (r/o)
  Base + 2   Control Register (r/w)
 
 
  Base + 0:
       Bits 0-7 Data being output to pins 2-9 (non-inverting).
       If read: if bit 5 of the control register is 0, you read the
       value that's previously been written (normal operation), otherwise
       the value read represents the data read from pins 2-9 [PS2 and
       some ATs only].
 
  Base + 1:
       Bits 0-2 unused
       Bit 3    "Error"; if 0, paper is out, the printer is off-line or an
                error occurred.
		Indicates the level of pin 15 (non-inv.)
       Bit 4    "Selected" if 1; most printers tie this signal high
       		Indicates the level of pin 13 (non-inv.)
       Bit 5    "Paper Out" if 1
       		Indicates the level of pin 12 (non-inv.)
       Bit 6    "Acknowledge" if 0 (normally for about 10ms)
       		Indicates the level of pin 10 (non-inv.)
       Bit 7    "Busy" if 0; printing is in progress or sth. like that
       		Indicates the level of pin 11 (inverted!)
 
  Base + 2:
       Bit 0    "Data Strobe"; set this to 1 for at least 1 microsecond to
                signal the printer that there is new data on the lines
                Controls pin 1; signal is inverted.
       Bit 1    "Autofeed XT"; if set, a LF is added to each CR by the
                printer. Controls pin 14; signal is inverted.
       Bit 2    "Init"; set this to 0 for at least 50us (Epson) to reset
                the printer. Controls pin 16; signal is not inverted.
       Bit 3    "Select input" (as seen by the printer); if 1, the codes
                "DC1" and "DC3" can activate and deactivate the printer.
		Controls pin 17. Signal is inverted.
       Bit 4    if 1: "Acknowledge" generates interrupt 0Fh (no external
                signal)
       Bit 5    [PS2 and some ATs] direction of the data lines. 0=output,
                1=input
       Bits 6-7 not used
 
       Normal setting in a PC: 0Ch
 
 
How to input data using the printer port
----------------------------------------
 
*** There is no possibility to switch the data lines of the printer port
to input, except for some computers. Try writing FFh to Base+0 and 2Ch to
Base+2, then short one of the data lines to ground, input Base+0 and see
if one of the bits is 0. If yes, you're lucky. It works with all PS2's
and some ATs.
 
But there's a workaround: use a parallel-in-serial-out register, connect the
serial output to one of the status lines and the control pins (latch, clock)
to two of the data lines. Then give the appropriate signals to the register
by writing the correct values to the data register and read in the data
bit-wise through the status register. This is not very fast, but cheap.
 
Or, even better: use a multiplexer (74LS157) with four of the status lines
(those four that are non-inverting). Control it with one of the numerous
outputs.


How to do direct output to the printer
--------------------------------------

1. Write your data to Base+0.
2. Write 0Dh to Base+2.
3. Make sure there's a delay of at least one microsecond :-) (your code
   will do that for you...)
4. Write 0Ch to Base+2.
5. Input from Base+1 and check if Bit 7 is 1. If not, wait for some time
   but make sure you break the loop with "time out" if it takes too long.
6. ... and repeat this whole procedure as often as you like.


Pinout
------

1     Strobe
2-9   Data 0-7
10    Acknowledge
11    Busy
12    Paper out
13    Select (this is an input!)
14    Autofeed XT
15    Error
16    Init
17    Select Input (this is an output!, name as seen by the printer)
18-25 Ground
 
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