IBM XT/AT COMPATIBLE, BI-DIRECTIONAL
AND EPP MODES
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.
DATA PORT
ADDRESS OFFSET = 00H
BIT 4 SLCT - PRINTER SELECTED STATUS
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.
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 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.
STATUS PORT
ADDRESS OFFSET = 01H
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.
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 7 nBUSY - nBUSY
BIT 0 TMOUT - TIME OUT
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.
This bit is valid in EPP mode only and indicates
that a 10 msec time out has occurred on the
EPP bus. A logic “0” 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 “1” 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 “0”.
Writing a “0” to this bit has no effect.
CONTROL PORT
ADDRESS OFFSET = 02H
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 1 and 2 - are not implemented as register
bits, during a read of the Printer Status Register
these bits are a low level.
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