Centronics Parallel Interface:  This interface, originally designed and used
by Centronics, Inc., was copied by many other printer manufacturers to
eventually become the default parallel standard for printers.  As with RS232
this is merely a standard.  Adherence to it is left totally to each vendor.

As with the RS232 interface, the parallel interface has 4 categories of 
leads, ground, data, control, and timing.  However, the shape of
connnector is different.  The Centronics parallel connector differs from
a DB25 connector by having 36 pins in an amphenol type connector. 
Eight of the leads are used to carry the bits of a character in parallel.

The parallel interface is unidirectional.  This means that data can be
sent in only one direction, from the computer to the printer.  Hence
software flow control is not appropriate.  Only hardware flow control is
used in a parallel interface.  Another difference from RS232 is that there
is the capability to indicate the reason for a busy condition, or
why the printer can't receive any more data.  The printer, by turning off/on
the correct lead, can indicate it is out of paper, or that it is offline.
The computer can also clear the printer's buffer, as well as
reinitialize the printer's control logic(reset).  Depending on the printer
this can be used to clear the different print modes, compressed, italics,
or any other special setups.

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