📄 nibble mode.htm
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<DIV align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000
size=2><B>Nibble Mode</B></FONT> </DIV>
<P align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000
size=1>The Nibble mode is the most common way to get reverse channel data
from a printer or peripheral. This mode is usually combined with the
Compatibility mode or a proprietary forward channel mode to create a
complete bi-directional channel. </FONT></P>
<P align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000
size=1>All of the standard parallel ports provide 5 lines from the
peripheral to the PC to be used for external status indications. Using
these lines, a peripheral can send a byte of data (8-bits) by sending 2
nibbles (4-bits) of information to the PC in two data transfer cycles.
Unfortunately, since the nACK line is generally used to provide a
peripheral interrupt, the bits used to transfer a nibble are not
conveniently packed into the byte defined by the Status register. For this
reason, the software must read the status byte and then manipulate the
bits in order to get a correct byte. </FONT></P>
<P align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000
size=1>Table 1 identifies the signal names for the Nibble mode. Figure 1
shows the basic data handshake for a nibble mode transfer from the
peripheral to the host. </FONT></P>
<H4 align=center><FONT face="Arial, Helvetica, sans-serif" color=#000000
size=1>Table 1 -- Nibble Mode Signals</FONT></H4>
<TABLE border=1>
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<TR>
<TH><FONT face=Arial color=#000000 size=2>SPP Signal</FONT></TH>
<TH align=middle><FONT face=Arial color=#000000 size=2>Nibble Mode
Name</FONT></TH>
<TH align=middle><FONT face=Arial color=#000000
size=2>In/Out</FONT></TH>
<TH><FONT face=Arial color=#000000 size=2>Description -- Signal
usage when in Nibble Mode data transfer</FONT></TH></TR>
<TR>
<TD><FONT face=Arial color=#000000 size=1>nSTROBE</FONT></TD>
<TD align=middle><FONT face=Arial color=#000000
size=1>nSTROBE</FONT></TD>
<TD align=middle><FONT face=Arial color=#000000 size=1>Out</FONT></TD>
<TD><FONT face=Arial color=#000000 size=1>Not used for reverse data
transfer</FONT></TD></TR>
<TR>
<TD><FONT face=Arial color=#000000 size=1>nAUTOFEED</FONT></TD>
<TD align=middle><FONT face=Arial color=#000000
size=1>HostBusy</FONT></TD>
<TD align=middle><FONT face=Arial color=#000000 size=1>Out</FONT></TD>
<TD><FONT face=Arial color=#000000 size=1>Host nibble mode handshake
signal. Set low to indicate host is ready for nibble. Set high to
indicate nibble has been received.</FONT></TD></TR>
<TR>
<TD><FONT face=Arial color=#000000 size=1>nSELECTIN</FONT></TD>
<TD align=middle><FONT face=Arial color=#000000
size=1>1284Active</FONT></TD>
<TD align=middle><FONT face=Arial color=#000000 size=1>Out</FONT></TD>
<TD><FONT face=Arial color=#000000 size=1>Set high when host is in a
1284 transfer mode.</FONT></TD></TR>
<TR>
<TD><FONT face=Arial color=#000000 size=1>nINIT</FONT></TD>
<TD align=middle><FONT face=Arial color=#000000
size=1>nINIT</FONT></TD>
<TD align=middle><FONT face=Arial color=#000000 size=1>Out</FONT></TD>
<TD><FONT face=Arial color=#000000 size=1>Not used for reverse data
transfer</FONT></TD></TR>
<TR>
<TD><FONT face=Arial color=#000000 size=1>nACK</FONT></TD>
<TD align=middle><FONT face=Arial color=#000000
size=1>PtrClk</FONT></TD>
<TD align=middle><FONT face=Arial color=#000000 size=1>In</FONT></TD>
<TD><FONT face=Arial color=#000000 size=1>Set low to indicate valid
nibble data, set high in response to HostBusy going
high.</FONT></TD></TR>
<TR>
<TD><FONT face=Arial color=#000000 size=1>BUSY</FONT></TD>
<TD align=middle><FONT face=Arial color=#000000
size=1>PtrBusy</FONT></TD>
<TD align=middle><FONT face=Arial color=#000000 size=1>In</FONT></TD>
<TD><FONT face=Arial color=#000000 size=1>Used for Data bit 3, then
7</FONT></TD></TR>
<TR>
<TD><FONT face=Arial color=#000000 size=1>PE</FONT></TD>
<TD align=middle><FONT face=Arial color=#000000
size=1>AckDataReq</FONT></TD>
<TD align=middle><FONT face=Arial color=#000000 size=1>In</FONT></TD>
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