
96
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.
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.
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 0 STROBE - STROBE
This bit is inverted and output onto the nSTROBE output.
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 2 nINIT - nINITIATE OUTPUT
This bit is output onto the nINIT output without inversion.
BIT 3 SLCTIN - PRINTER SELECT INPUT
This bit is inverted and output onto the nSLCTIN output. A logic 1 on this bit selects the printer; a
logic 0 means the printer is not selected.
BIT 4 IRQE - INTERRUPT REQUEST ENABLE
The interrupt request enable bit when set to a high level may be used to enable interrupt requests from
the Parallel Port to the CPU. An interrupt request is generated on the IRQ port by a positive going
nACK input. When the IRQE bit is programmed low the IRQ is disabled.
BIT 5 PCD - PARALLEL CONTROL DIRECTION
Parallel Control Direction is not valid in printer mode. In printer mode, the direction is always out
regardless of the state of this bit. In bi-directional, EPP or ECP mode, a logic 0 means that the printer
port is in output mode (write); a logic 1 means that the printer port is in input mode (read).
Bits 6 and 7 during a read are a low level, and cannot be written.
EPP Address Port
ADDRESS OFFSET = 03H
The EPP Address Port is located at an offset of '03H' from the base address. The address register
is cleared at initialization by RESET. During a WRITE operation, the contents of the internal data bus
DB0-DB7 are buffered (non inverting) and output onto the PD0 - PD7 ports. An LPC I/O write cycle
causes an EPP ADDRESS WRITE cycle to be performed, during which the data is latched for the