The level on the SLCT input is read by the CPU as
bit 4 of the Printer Status Register. A logic 1
means the printer is on line; a logic 0 means it is
not selected.
IBM XT/AT COMPATIBLE, BI-
DIRECTIONAL AND EPP MODES
DATA PORT
ADDRESS OFFSET = 00H
BIT 5 PE - PAPER END
The level on the PE input is read by the CPU as bit
5 of the Printer Status Register. A logic 1
indicates a paper end; a logic 0 indicates the
presence of paper.
The Data Port is located at an offset of '00H' from
the base address. The data register is cleared at
initialization by RESET.
During a WRITE
operation, the Data Register latches the contents
of the data bus with the rising edge of the nIOW
input. The contents of this register are buffered
(non inverting) and output onto the PD0 - PD7
ports. During a READ operation in SPP mode,
PD0 - PD7 ports are buffered (not latched) and
output to the host CPU.
BIT 6 nACK - nACKNOWLEDGE
The level on the nACK input is read by the CPU as
bit 6 of the Printer Status Register. A logic 0
means that the printer has received a character
and can now accept another. A logic 1 means that
it is still processing the last character or has not
received the data.
STATUS PORT
ADDRESS OFFSET = 01H
BIT 7 nBUSY - nBUSY
The complement of the level on the BUSY input is
read by the CPU as bit 7 of the Printer Status
Register. A logic 0 in this bit means that the
printer is busy and cannot accept a new character.
A logic 1 means that it is ready to accept the next
character.
The Status Port is located at an offset of '01H'
from the base address. The contents of this
register are latched for the duration of an nIOR
read cycle. The bits of the Status Port are defined
as follows:
BIT 0 TMOUT - TIME OUT
CONTROL PORT
ADDRESS OFFSET = 02H
This bit is valid in EPP mode only and indicates
that a 10 usec time out has occurred on the EPP
bus. A logic O means that no time out error has
occurred; a logic 1 means that a time out error has
been detected. This bit is cleared by a RESET.
Writing a one to this bit clears the time out status
bit. On a write, this bit is self clearing and does not
require a write of a zero. Writing a zero to this bit
has no effect.
The Control Port is located at an offset of '02H'
from the base address. The Control Register is
initialized by the RESET input, bits 0 to 5 only
being affected; bits 6 and 7 are hard wired low.
BIT 0 STROBE - STROBE
This bit is inverted and output onto the nSTROBE
output.
BITS 1, 2 - are not implemented as register bits,
during a read of the Printer Status Register these
bits are a low level.
BIT 1 AUTOFD - AUTOFEED
This bit is inverted and output onto the nAUTOFD
output. A logic 1 causes the printer to generate a
line feed after each line is printed. A logic 0 means
no autofeed.
BIT 3 nERR - nERROR
The level on the nERROR input is read by the
CPU as bit 3 of the Printer Status Register.
A
logic 0 means an error has been detected; a logic
1 means no error has been detected.
BIT 4 SLCT - PRINTER SELECTED STATUS
88
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