PC parallel LPT port pinout

Parallel port will allow the input of up to 9 bits or the output of 12 bits at any one given time. This port may be used for interfacing home made projects since external circuitry is minimal for many simple tasks. The port is composed of 4 control lines, 5 status lines and 8 data lines. It's found commonly on the back of your PC as a 25 Pin D-SUB female connector (note, that 25 pin D-SUB male connector represent RS-232 serial port, not compatible with LPT!).

There are differnt modes of Parallel port work in modern computer. Take a look to newer ECP Parallel LPT port (IEEE-1284A) interface for more detailed explanations. Information included in current page is about older, but still widely accepted SPP LPT port interface. ECP specification includes SPP as one of possible modes.

Pin	Name	Dir	Description
1	/STROBE		Strobe
2	D0		Data Bit 0
3	D1		Data Bit 1
4	D2		Data Bit 2
5	D3		Data Bit 3
6	D4		Data Bit 4
7	D5		Data Bit 5
8	D6		Data Bit 6
9	D7		Data Bit 7
10	/ACK		Acknowledge
11	BUSY		Busy
12	PE		Paper End
13	SEL		Select
14	/AUTOFD		Autofeed
15	/ERROR		Error
16	/INIT		Initialize
17	/SELIN		Select In
18	GND		Signal Ground
19	GND		Signal Ground
20	GND		Signal Ground
21	GND		Signal Ground
22	GND		Signal Ground
23	GND		Signal Ground
24	GND		Signal Ground
25	GND		Signal Ground

The data output of the Parallel Port is normally TTL logic levels. Most Parallel Ports implemented in ASIC, can sink and source around 12mA. However voer variations possible: Sink/Source 6mA, Source 12mA/Sink 20mA, Sink 16mA/Source 4mA, Sink/Source 12mA and others.

Centronics is an early used standard for transferring data from a host to the printer. The majority of printers use this handshake.

               ______          ___________________
 nStrobe             \        /
                      \______/
                       ______________
 Busy                 /              \
               ______/                \___________
               ______________________         ____
 nAck                                \       /
                                      \_____/
               ___    _______    _________________
                  \  /       \  /
 Data              \/         \/
                   /\         /\
               __ /  \_______/  \_________________
                                                 

Data is first applied on the Parallel Port pins 2 to 7. The host then checks to see if the printer is busy. i.e. the busy line should be low. The program then asserts the strobe, waits a minimum of 1mS, and then de-asserts the strobe. Data is normally read by the printer/peripheral on the rising edge of the strobe. The printer will indicate that it is busy processing data via the Busy line. Once the printer has accepted data, it will acknowledge the byte by a negative pulse about 5mS on the nAck line. Host may ignore the nAck line to save time.

Note: Direction is Computer relative Device.