
106
BIT 3 SELECTIN
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 ackIntEn - 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 due to a low to high transition on the nACK input. Refer to the
description of the interrupt under Operation, Interrupts.
BIT 5 DIRECTION
If mode=000 or mode=010, this bit has no effect and the direction is always out regardless of the state
of this bit. In all other modes, Direction is valid and 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.
cFifo (Parallel Port Data FIFO)
ADDRESS OFFSET = 400h
Mode = 010
Bytes written or DMAed from the system to this FIFO are transmitted by a hardware handshake to the
peripheral using the standard parallel port protocol. Transfers to the FIFO are byte aligned. This
mode is only defined for the forward direction.
ecpDFifo (ECP Data FIFO)
ADDRESS OFFSET = 400H
Mode = 011
Bytes written or DMAed from the system to this FIFO, when the direction bit is 0, are transmitted by a
hardware handshake to the peripheral using the ECP parallel port protocol. Transfers to the FIFO are
byte aligned.
Data bytes from the peripheral are read under automatic hardware handshake from ECP into this
FIFO when the direction bit is 1. Reads or DMAs from the FIFO will return bytes of ECP data to the
system.
tFifo (Test FIFO Mode)
ADDRESS OFFSET = 400H
Mode = 110
Data bytes may be read, written or DMAed to or from the system to this FIFO in any direction. Data in
the tFIFO will not be transmitted to the to the parallel port lines using a hardware protocol handshake.
However, data in the tFIFO may be displayed on the parallel port data lines.
The tFIFO will not stall when overwritten or underrun. If an attempt is made to write data to a full
tFIFO, the new data is not accepted into the tFIFO. If an attempt is made to read data from an empty
tFIFO, the last data byte is re-read again. The full and empty bits always keep track of the correct
FIFO state. The tFIFO will transfer data at the maximum ISA rate so that software may generate
performance metrics.