Summary of pins of PC/AT communication ports

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AT Serial 9 pins

   1 	CD

   2	RxD

   3	TxD

   4	DTR

   5	GND

   6	DSR

   7	RTS

   8	CTS

   9	RI

AT Serial 25 pins

   2	TxD

   3	RxD

   4	RTS

   5	CTS

   6	DSR

   7	GND

   8	CD

  20	DTR

  22	RI

Parallel Connector

   1	strobe

   2	data 0

   3	data 1

   .	...

   9	data 7

  10	acknowledge

  11	busy

  12	out of paper

  13	select

  14	auto feed

  15	error

  16	init printer

  17	select input

  18	ground

  19	ground

   .	...

  25	ground