PRINTER HANDSHAKING

 XON/XOFF Protocol
                  CABLE pin used: TD ( pin-2 )
                                  RD ( pin-3 )
                          Signal Gnd ( pin-7 )

     - When printer's buffer is full, it sends an XOFF character ( CTRL-S )
       to tell the system to stop sending job's data momentarily.

     - Printer continues to print buffer's content. When the buffer becomes
       low, the printer sends a XON character ( CTRL-Q ) to tell the system
       to send job's data again.
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 DTR Protocol
      CABLE pins used: TD, RD, CTS, RTS, CD ( or DSR ), DTR and Signal Gnd

         - System CTS detects if printer is power on ( RTS)
         - System CD detects if printer is on-line ( READY )
         - Printer drop DTR when it is not ready to receive data.
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 DEV_WAIT or DEV_BUSY ???

    The "DEV_WAIT" Status is set when the queue is waiting on the
    printer, because the printer is: off line, paper out, jammed paper,
    bad cable, lose cable, etc...

    The "DEV_BUSY" Status is set when there are more than one queue
    defined for a printer device file (/dev/lp0), and  another queue
    is currently using the printer device file.