📄 parallel port background.htm
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<TD><FONT face="Arial, Helvetica, sans-serif" size=1>BUSY</FONT></TD>
<TD><FONT face="Arial, Helvetica, sans-serif" size=1>In</FONT></TD>
<TD><FONT face="Arial, Helvetica, sans-serif" size=1>A high signal
asserted by the printer to indicate that it is busy and cannot take
data.</FONT></TD></TR>
<TR>
<TD> </TD>
<TD><FONT face="Arial, Helvetica, sans-serif" size=1>PE</FONT></TD>
<TD><FONT face="Arial, Helvetica, sans-serif" size=1>In</FONT></TD>
<TD><FONT face="Arial, Helvetica, sans-serif" size=1>Paper
Empty</FONT></TD></TR>
<TR>
<TD> </TD>
<TD><FONT face="Arial, Helvetica, sans-serif"
size=1>SELECT</FONT></TD>
<TD><FONT face="Arial, Helvetica, sans-serif" size=1>In</FONT></TD>
<TD><FONT face="Arial, Helvetica, sans-serif" size=1>Asserted high
to indicate that the printer is online.</FONT></TD></TR>
<TR>
<TD> </TD>
<TD><FONT face="Arial, Helvetica, sans-serif"
size=1>nERROR</FONT></TD>
<TD><FONT face="Arial, Helvetica, sans-serif" size=1>In</FONT></TD>
<TD><FONT face="Arial, Helvetica, sans-serif" size=1>Asserted low to
indicate that some error condition exists</FONT></TD></TR>
<TR>
<TD><FONT face="Arial, Helvetica, sans-serif" size=1>Data</FONT></TD>
<TD><FONT face="Arial, Helvetica, sans-serif"
size=1>DATA[8:1]</FONT></TD>
<TD><FONT face="Arial, Helvetica, sans-serif" size=1>Out</FONT></TD>
<TD><FONT face="Arial, Helvetica, sans-serif" size=1>8 data lines-
output only in older SPP</FONT></TD></TR></TBODY></TABLE></DIV>
<P align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000
size=1><SUP>(Note 1)</SUP> The signal usage described in this and all
following tables define the usage while in the described data transfer
mode. Many of these signals are used for mode transitions and for
additional status information. Please refer to the IEEE 1284-1994 standard
for the complete definition and usage of these signals. This is meant as
an introduction only. </FONT></P>
<H4 align=center><FONT face="Arial, Helvetica, sans-serif" color=#000000
size=1>Table 2 -- SPP Register Definition</FONT></H4>
<DIV align=left>
<TABLE border=1>
<TBODY>
<TR>
<TH align=middle><FONT face="Arial, Helvetica, sans-serif"
size=2>Register Offset<SUP>(2)</SUP></FONT></TH>
<TH align=middle><FONT face="Arial, Helvetica, sans-serif"
size=2>Name</FONT></TH>
<TH align=middle><FONT face="Arial, Helvetica, sans-serif"
size=2>Read/Write</FONT></TH>
<TH><FONT face="Arial, Helvetica, sans-serif"
size=2>Description</FONT></TH></TR>
<TR>
<TD align=middle><FONT face="Arial, Helvetica, sans-serif"
size=1>0</FONT></TD>
<TD><FONT face="Arial, Helvetica, sans-serif" size=1>Data
Register</FONT></TD>
<TD align=middle><FONT face="Arial, Helvetica, sans-serif"
size=1>R/W</FONT></TD>
<TD><FONT face="Arial, Helvetica, sans-serif" size=1>Data port to
read or write data</FONT></TD></TR>
<TR>
<TD align=middle><FONT face="Arial, Helvetica, sans-serif"
size=1>1</FONT></TD>
<TD><FONT face="Arial, Helvetica, sans-serif" size=1>Status
Register</FONT></TD>
<TD align=middle><FONT face="Arial, Helvetica, sans-serif"
size=1>R</FONT></TD>
<TD><FONT face="Arial, Helvetica, sans-serif" size=1>Contains status
bits</FONT></TD></TR>
<TR>
<TD align=middle><FONT face="Arial, Helvetica, sans-serif"
size=1>2</FONT></TD>
<TD><FONT face="Arial, Helvetica, sans-serif" size=1>Control
Register</FONT></TD>
<TD align=middle><FONT face="Arial, Helvetica, sans-serif"
size=1>W</FONT></TD>
<TD><FONT face="Arial, Helvetica, sans-serif" size=1>Used to set
control signals</FONT></TD></TR>
<TR>
<TD align=middle><FONT face="Arial, Helvetica, sans-serif"
size=1>3-7</FONT></TD>
<TD><FONT face="Arial, Helvetica, sans-serif"
size=1>Various</FONT></TD>
<TD align=middle><FONT face="Arial, Helvetica, sans-serif"
size=1>N/A</FONT></TD>
<TD><FONT face="Arial, Helvetica, sans-serif" size=1>Used
differently by various
implementations</FONT></TD></TR></TBODY></TABLE></DIV>
<P align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000
size=1><SUP>(Note 2)</SUP> This is the offset from the base address of the
port. </FONT></P>
<H3 align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000
size=2><B>IEEE 1284 Data Transfer Modes</B></FONT></H3>
<P align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000
size=1>The use of the various 1284 data transfer modes provide the
capability to create a forward and reverse channel connection between the
host computer and an attached peripheral. Since there is only one set of
data lines the connection is half duplex, data is transferred in one
direction at a time. </FONT></P>
<P align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000
size=1>The Compatibility and Nibble modes of operation can be implemented
in any existing parallel port in order to create a complete bi-directional
communication path between the host and peripheral. The Compatibility and
Byte modes can also be used create a bi-directional communication path,
but the the parallel port must support the Byte mode capability. The Byte
mode requires that an entire byte of data can be read from the external
data lines. This is usually implemented by the addition of a direction bit
in the parallel port's Control register. This type of port is generally
called a "bi-directional" parallel port. </FONT></P>
<P align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000
size=1>The EPP and ECP modes have bi-directional capability as part of
their protocol. These modes require that the hardware implement a state
machine that is capable of automatically generating the control strobes
that are necessary for these high performance data transfer modes.
</FONT></P>
<P align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000
size=1>Each of the operating modes, other than Compatibility, rename the
control and status signals to have meaning within the mode being used. The
discussions for each mode will use the names consistent with the mode
being discussed. <BR><BR></FONT></P>
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