nibble mode.htm

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<HTML><HEAD><TITLE>Warp Nine Engineering - The IEEE 1284 Experts - IEEE 1284 Nibble Mode</TITLE>
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      <P><IMG height=77 src="Nibble Mode.files/spacer2.gif" width=55></P>
      <P>&nbsp;</P></TD>
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      <P>&nbsp;</P>
      <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>
        <TBODY>
        <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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