requests from the Parallel Port to the CPU due to a low to
high transition on the nACK input. Refer to the
tFifo (Test FIFO Mode)
ADDRESS OFFSET = 400H
description of the interrupt under Operation, Interrupts.
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.
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).
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 must 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.
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.
The FIFO size and interrupt threshold can be determined
by writing bytes to the FIFO and checking the full and
serviceIntr bits.
The writeIntrThreshold can be determined by starting with
a full tFIFO, setting the direction bit to 0 and emptying it a
byte at a time until serviceIntr is set. This may generate
a spurious interrupt, but will indicate that the threshold
has been reached.
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.
The readIntrThreshold can be determined by setting the
direction bit to 1 and filling the empty tFIFO a byte at a
time until serviceIntr is set. This may generate a
spurious interrupt, but will indicate that the threshold has
been reached.
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.
100
SMSC [ SMSC CORPORATION ]
