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.
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 > And in case you forgot, it plugs into what is called the
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