nibble mode.htm

并口的接口规范

          <TD><FONT face=Arial color=#000000 size=1>Used for Data bit 2, then 
            6</FONT></TD></TR>
        <TR>
          <TD><FONT face=Arial color=#000000 size=1>SELECT</FONT></TD>
          <TD align=middle><FONT face=Arial color=#000000 
          size=1>Xflag</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 1, then 
            5</FONT></TD></TR>
        <TR>
          <TD><FONT face=Arial color=#000000 size=1>nERROR</FONT></TD>
          <TD align=middle><FONT face=Arial color=#000000 
            size=1>nDataAvail</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 0, then 
            4</FONT></TD></TR>
        <TR>
          <TD><FONT face=Arial color=#000000 size=1>DATA[8:1]</FONT></TD>
          <TD align=middle><FONT face=Arial color=#000000 size=1>Not 
            Used</FONT></TD>
          <TD><FONT size=1>&nbsp;</FONT></TD></TR></TBODY></TABLE>
      <P align=center><FONT face="Arial, Helvetica, sans-serif" color=#000000 
      size=1><IMG height=168 src="Nibble Mode.files/nibfig1.gif" 
      width=488><BR></FONT></P>
      <H3 align=center><FONT face="Arial, Helvetica, sans-serif" color=#000000 
      size=1>Figure 1 -- Nibble Mode Data Transfer Cycle <STRONG>1284 Nibble 
      Mode phase transition:</STRONG> </FONT></H3>
      <OL>
        <LI>
        <DIV align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000 
        size=1>Host signals ability to take data by asserting HostBusy 
        low</FONT></DIV>
        <LI>
        <DIV align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000 
        size=1>Peripheral responds by placing first nibble on status 
        lines</FONT></DIV>
        <LI>
        <DIV align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000 
        size=1>Peripheral signals valid nibble by asserting PtrClk 
        low</FONT></DIV>
        <LI>
        <DIV align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000 
        size=1>Host sets HostBusy high to indicate that it has received the 
        nibble and is not yet ready for another nibble.</FONT></DIV>
        <LI>
        <DIV align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000 
        size=1>Peripheral sets PtrClk high to acknowledge host</FONT></DIV>
        <LI>
        <DIV align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000 
        size=1>States 1 through 5 repeat for the second 
      nibble</FONT></DIV></LI></OL>
      <P align=left><FONT face="Arial, Helvetica, sans-serif" color=#000000 
      size=1>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. </FONT><FONT face=Arial 
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