Table 43 - Forward Channel Commands (HostAck Low)
Reverse Channel Commands (PeripAck Low)
D7
D[6:0]
0
Run-Length Count (0-127)
(mode 0011 0X00 only)
1
Channel Address (0-127)
Pin Definition
Data Compression
The drivers for nStrobe, nAutoFd, nInit and
nSelectIn are open-collector in mode 000 and
are push-pull in all other modes.
The ECP port supports run length encoded
(RLE) decompression in hardware and can
transfer compressed data to a peripheral. Run
length encoded (RLE) compression in hardware
is not supported. To transfer compressed data
in ECP mode, the compression count is written
to the ecpAFifo and the data byte is written to
the ecpDFifo.
ISA Connections
The interface can never stall causing the host to
hang. The width of data transfers is strictly
controlled on an I/O address basis per this
specification. All FIFO-DMA transfers are byte
wide, byte aligned and end on a byte boundary.
(The PWord value can be obtained by reading
Configuration Register A, cnfgA, described in
the next section). Single byte wide transfers
are always possible with standard or PS/2
mode using program control of the control
signals.
Compression is accomplished by counting
identical bytes and transmitting an RLE byte
that indicates how many times the next byte is
to be repeated. Decompression simply
intercepts the RLE byte and repeats the
following byte the specified number of times.
When a run-length count is received from a
peripheral, the subsequent data byte is
replicated the specified number of times. A
run-length count of zero specifies that only one
byte of data is represented by the next data
byte, whereas a run-length count of 127
indicates that the next byte should be expanded
to 128 bytes. To prevent data expansion,
however, run-length counts of zero should be
avoided.
Interrupts
The interrupts are enabled by serviceIntr in the
ecr register.
serviceIntr = 1 Disables the DMA and all of the
service interrupts.
serviceIntr = 0 Enables the selected interrupt
condition. If the interrupting
condition is valid, then the
interrupt
is
generated
immediately when this bit is
changed from a “1” to a “0”.
This
can
occur
during
Programmed I/O if the number
of bytes removed or added
99
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