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SPP Signal
Nibble
Mode NameIn/Out
Description
Signal usage when in Nibble Mode data transfer
nSTROBE
nSTROBE
Out
Not used for reverse data transfer
nAUTOFEED
HostBusy
Out
Host nibble mode handshake signal.
Set low to indicate host is ready for nibble.
Set high to indicate nibble has been received.
nSELECTIN
1284Active
Out
Set high when host is in a 1284 transfer mode.
nINIT
nINIT
Out
Not used for reverse data transfer
nACK
PtrClk
In
Set low to indicate valid nibble data, set high
in response to HostBusy going high.
BUSY
PtrBusy
In
Used for Data bit 3, then 7
PE
AckDataReq
In
Used for Data bit 2, then 6
SELECT
Xflag
In
Used for Data bit 1, then 5
nERROR
nDataAvail
In
Used for Data bit 0, then 4
DATA[8:1]
Not Used
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1284 Nibble Mode phase transistion
Nibble mode, like the Compatible mode, requires that the software drive the
protocol by setting and reading the lines on the parallel port. Nibble mode is the most
software intensive mode for reverse channel data communication. For this reason, there is
a severe limitation of approximately 50K bytes per second for this type of data transfer.
The major advantage of this combination of modes is the ability to operate on all PCs that
have a parallel port. The performance limitations incurred by the Nibble mode operation
does not have much visible effect on peripherals that have low reverse channel
requirements, such as printers, but can be nearly intolerable when used for LAN adapters,
disk drives or CD ROM drives.
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