Parallel port pins 18-25

Most PC parallel port literature seems to state that pins 18-25 are
connected to ground, however, I have a laptop with parallel port pin 25
not connected to anything IMO. At first I thought it was a faulty cable
but that wasn't the case. Pins 18-24 are all connected to ground.
What's the deal here?

 

Historically, pin 25, was reserved for the parity bit on one high speed
IBM printer connection, on the first port only. This was specific to a
couple of IBM machines, and 99% of other people never implemented this.
The line is either left N/C (floating), or connected to ground. The latter
IEEE specification for the port has the line used as the ground connection
for three of the handshake lines. Normally the ground connections are all
simply shorted together, so having the pin N/C, should not be a problem.
The reason is commonly that a 24pin PCB header has been used to a ribbon
cable, feeding the rear plug, so the manufacturers leave this ground
unconnected.

Best Wishes

 

The reason is commonly that a 24pin PCB header has been used to a ribbon

Thanks, makes sense.

 

Some implementations of the Centronix interface used pin 25
from printer as a +5V power source with limited current.

Remember one side of the standard IBM cable is a 25 pin
connector. The other side is a 36 pin connector. Therefore
all 25 pins on the first connector would be connected to a
each wire in the ribbon cable. But when talking about cables,
first define with connector is being used - the 25 pin D
connector or the 36 pin Centronix connector.

 

Remember one side of the standard IBM cable is a 25 pin

And in case you forgot, it plugs into what is called the parallel port which
leads to the subject above and what was being discussed until your post.

 

Centronix parallel port. The signal called Error is pin 32
on the printer; pin 15 on the PC. The signal called
Initialize Printer is pin 31 on the printer; pin 16 on the
PC. Both ends of the cable are standard parallel port
connectors. And yet pin numbers are different on both ends of
the cable. I did not forget anything. The pin numbered 18 on
the printer parallel port connector sometimes connected to pin
25 on the parallel port connector that might connect to PC.

 

Some laptops can connect an external floppy drive to
the parallel port connector.

It's an alternative function. Switching between parport
and floppy function could be done by pulling high an
'unconnected' ground pin.

Regards,

Iwo