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<HTML><HEAD><TITLE>Warp Nine Engineering - The IEEE 1284 Experts - IEEE 1284 Byte Mode</TITLE>
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    <TD vAlign=top align=left width=1 bgColor=#ff0000 height=297>&nbsp; </TD>
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      <H1 align=left><FONT color=#ffff00><BR><FONT 
      face="Arial, Helvetica, sans-serif" color=#000000 size=2><B>Byte 
      Mode</B></FONT></FONT></H1>
      <P align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000 
      size=1>With later implementations of the parallel port interface, some 
      manufacturers, led by IBM on the PS/2 parallel port, added the capability 
      to disable the drivers used for driving the data lines, and allowed the 
      data port to become an input read data port. This enables a peripheral to 
      send an entire byte of data to the PC in one data transfer cycle by using 
      the 8 data lines, rather than the two cycles required using the Nibble 
      mode. </FONT></P>
      <P align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000 
      size=1>This ability enables a Byte mode for reverse channel data transfer 
      that can be used to provide data rates into the PC approaching that of the 
      Compatibility mode, from the PC. This type of port is sometimes referred 
      to as a "enhanced bi-directional" port, and has caused some confusion when 
      mistaken for an Enhanced Parallel Port (EPP). </FONT></P>
      <P align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000 
      size=1>The following table identifies the Byte mode signal names, and 
      figure 3 shows the handshake for a Byte mode data transfer. </FONT></P>
      <H4 align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000 
      size=1>Table 1 -- Byte Mode Signals</FONT></H4>
      <DIV align=left>
      <TABLE border=1>
        <TBODY>
        <TR>
          <TH><FONT face="Arial, Helvetica, sans-serif" size=2>SPP 
            Signal</FONT></TH>
          <TH><FONT face="Arial, Helvetica, sans-serif" size=2>Byte Mode 
            Name</FONT></TH>
          <TH align=middle><FONT face="Arial, Helvetica, sans-serif" 
            size=2>In/Out</FONT></TH>
          <TH>
            <P align=center><FONT face="Arial, Helvetica, sans-serif" 
            size=2>Description<BR>Signal usage when in Byte Mode data 
            transfer</FONT> </P></TH></TR>
        <TR>
          <TD><FONT face="Arial, Helvetica, sans-serif" 
          size=1>nSTROBE</FONT></TD>
          <TD><FONT face="Arial, Helvetica, sans-serif" 
          size=1>HostClk</FONT></TD>
          <TD align=middle><FONT face="Arial, Helvetica, sans-serif" 
            size=1>Out</FONT></TD>
          <TD><FONT face="Arial, Helvetica, sans-serif" size=1>Pulsed low at 
            the end of each Byte mode data transfer to indicate that the byte 
            was received. Acknowledge signal.</FONT></TD></TR>
        <TR>
          <TD><FONT face="Arial, Helvetica, sans-serif" 
          size=1>nAUTOFEED</FONT></TD>
          <TD><FONT face="Arial, Helvetica, sans-serif" 
          size=1>HostBusy</FONT></TD>
          <TD align=middle><FONT face="Arial, Helvetica, sans-serif" 
            size=1>Out</FONT></TD>
          <TD><FONT face="Arial, Helvetica, sans-serif" size=1>Set low to 
            indicate host is ready for byte. Set high to indicate byte has been 
            received. Handshake signal.</FONT></TD></TR>
        <TR>
          <TD><FONT face="Arial, Helvetica, sans-serif" 
          size=1>nSELECTIN</FONT></TD>
          <TD><FONT face="Arial, Helvetica, sans-serif" 
            size=1>1284Active</FONT></TD>
          <TD align=middle><FONT face="Arial, Helvetica, sans-serif" 
            size=1>Out</FONT></TD>
          <TD><FONT face="Arial, Helvetica, sans-serif" size=1>Set high when 
            host is in a 1284 transfer mode.</FONT></TD></TR>
        <TR>
          <TD><FONT face="Arial, Helvetica, sans-serif" size=1>nINIT</FONT></TD>
          <TD><FONT face="Arial, Helvetica, sans-serif" size=1>nINIT</FONT></TD>
          <TD align=middle><FONT face="Arial, Helvetica, sans-serif" 
            size=1>Out</FONT></TD>
          <TD><FONT face="Arial, Helvetica, sans-serif" size=1>Not used. Set 
            high.</FONT></TD></TR>
        <TR>
          <TD><FONT face="Arial, Helvetica, sans-serif" size=1>nACK</FONT></TD>
          <TD><FONT face="Arial, Helvetica, sans-serif" 
size=1>PtrClk</FONT></TD>
          <TD align=middle><FONT face="Arial, Helvetica, sans-serif" 
            size=1>In</FONT></TD>
          <TD><FONT face="Arial, Helvetica, sans-serif" size=1>Set low to 
            indicate valid data on the data lines, set high in response to 
            HostBusy going high.</FONT></TD></TR>
        <TR>
          <TD><FONT face="Arial, Helvetica, sans-serif" size=1>BUSY</FONT></TD>
          <TD><FONT face="Arial, Helvetica, sans-serif" 
          size=1>PtrBusy</FONT></TD>
          <TD align=middle><FONT face="Arial, Helvetica, sans-serif" 
            size=1>In</FONT></TD>
          <TD><FONT face="Arial, Helvetica, sans-serif" size=1>Forward channel 
            Busy status.</FONT></TD></TR>
        <TR>
          <TD><FONT face="Arial, Helvetica, sans-serif" size=1>PE</FONT></TD>
          <TD><FONT face="Arial, Helvetica, sans-serif" 
            size=1>AckDataReq</FONT></TD>
          <TD align=middle><FONT face="Arial, Helvetica, sans-serif" 
            size=1>In</FONT></TD>
          <TD><FONT face="Arial, Helvetica, sans-serif" size=1>Follows 
            nDataAvail</FONT></TD></TR>
        <TR>
          <TD><FONT face="Arial, Helvetica, sans-serif" 
size=1>SELECT</FONT></TD>
          <TD><FONT face="Arial, Helvetica, sans-serif" size=1>Xflag</FONT></TD>
          <TD align=middle><FONT face="Arial, Helvetica, sans-serif" 
            size=1>In</FONT></TD>
          <TD><FONT face="Arial, Helvetica, sans-serif" size=1>Extensibility 
            flag. Not used in Byte mode.</FONT></TD></TR>
        <TR>
          <TD><FONT face="Arial, Helvetica, sans-serif" 
size=1>nERROR</FONT></TD>
          <TD><FONT face="Arial, Helvetica, sans-serif" 
            size=1>nDataAvail</FONT></TD>
          <TD align=middle><FONT face="Arial, Helvetica, sans-serif" 
            size=1>In</FONT></TD>
          <TD><FONT face="Arial, Helvetica, sans-serif" size=1>Set low by 
            peripheral to indicate that reverse data is available.</FONT></TD></TR>
        <TR>
          <TD><FONT face="Arial, Helvetica, sans-serif" 
          size=1>DATA[8:]</FONT></TD>
          <TD><FONT face="Arial, Helvetica, sans-serif" 
          size=1>DATA[8:1]</FONT></TD>
          <TD align=middle><FONT face="Arial, Helvetica, sans-serif" 
            size=1>Bi-Di</FONT></TD>
          <TD><FONT face="Arial, Helvetica, sans-serif" size=1>Used to provide 
            data from peripheral to host.</FONT></TD></TR></TBODY></TABLE></DIV>
      <P align=center><FONT face="Arial, Helvetica, sans-serif" color=#000000 
      size=1><BR><BR><IMG height=230 src="Byte Mode.files/bmdfig1.jpg" width=460 
      align=middle><BR><BR>Figure 1 -- Byte Mode Data Transfer Cycle</FONT></P>
      <P align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000 
      size=1><BR>Byte Mode signal transitions: </FONT></P>
      <P align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000 
      size=1>1. Host signals ability to take data by asserting HostBusy 
      low</FONT></P>
      <P align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000 
      size=1>2. Peripheral responds by placing first byte on data 
      lines</FONT></P>
      <P align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000 
      size=1>3. Peripheral signals valid byte by asserting PtrClk low</FONT></P>
      <P align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000 
      size=1>4. Host sets HostBusy high to indicate that it has received the and 
      is not yet ready for another byte</FONT></P>
      <P align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000 
      size=1>5. Peripheral sets PtrClk high to acknowledge host. Host pulses 
      HostClk as an acknowledgement to the peripheral</FONT></P>
      <P align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000 
      size=1>6. States 1 through 5 repeat for additional bytes</FONT></P>
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