Digital Electronics: Pinout Assignments for a Parallel Port

By Doug Lowe from Electronics All-In-One Desk Reference For Dummies

The pins on a parallel port that you’re most interested in for your electronics projects are pins 2 through 9 — the eight data pins that are collectively called the data port.

When the data port is connected to a printer, its eight pins are capable of sending 1 byte of data at a time to the printer. When the data port is connected to a circuit of your own design, its pins operate as eight separate logic outputs, which you can use as inputs to your own logic circuits.

A parallel port also features four additional output pins called the control port, which you can also use for output.

When the control port is connected to a printer, these pins are used to control the operation of the printer. One of them, called the strobe, indicates that a new byte of data is available on the data pins; when the strobe pin goes HIGH, the printer reads a byte of data from the data pins. Another control-port pin resets the printer.

Finally, the five pins that make up the status port allow the printer to send information back to the computer.

One of the status-port pins lets the printer tell the computer that it’s ready to receive data via the data port. Another pin lets the printer know that it has finished reading data from the data port. A third pin informs the computer that the printer is out of paper. The other status pins have similar functions.

Pin	Name	Input or Output	Description
1	STROBE	Output or Input	LOW when data is present on the data pins
2	D0	Output	Data bit 0
3	D1	Output	Data bit 1
4	D2	Output	Data bit 2
5	D3	Output	Data bit 3
6	D4	Output	Data bit 4
7	D5	Output	Data bit 5
8	D6	Output	Data bit 6
9	D7	Output	Data bit 7
10	ACK	Input	LOW when data has been read
11	BUSY	Input	HIGH when the printer is busy
12	PE	Input	HIGH when the printer is out of paper
13	SEL	Input	HIGH when the printer is ready
14	LINEFEED	Output or Input	Advances the printer
15	ERROR	Input	HIGH when an error condition exists
16	RESET	Output or Input	HIGH when the printer is reset
17	SELECT	Output or Input	HIGH when the printer is offline
18	GND0	Neither	Ground connection
19	GND1	Neither	Ground connection
20	GND2	Neither	Ground connection
21	GND3	Neither	Ground connection
22	GND4	Neither	Ground connection
23	GND5	Neither	Ground connection
24	GND6	Neither	Ground connection
25	GND7	Neither	Ground connection

As I’ve already mentioned, the output pins of a parallel port use a +5 V HIGH signal to represent 1 and 0 V to represent 0. The amount of current that each pin can source is relatively small — typically, around 10–12 mA.

That current is enough to drive an LED, but for anything more demanding, you need a way to isolate the output load from the parallel port itself. To do that, you can use individual transistors or an IC designed specifically for this purpose.