The Home of Electronic Engineering and Embedded Systems Programming

The standard PC printer port is handy for testing and controlling devices. It provides an easy way to implement a small amount of digital I/O. I like to use to during initial development of a product -- before the "real" hardware is ready, I can dummy up a circuit using the printer port, and thus get started testing my software.

This source code module provides the low-level control of the port, implementing code to control 12 outputs and read 5 inputs.

This code was written for Borland C/C++ v3.1, but you should be able to adapt it for other compilers. You can view the source code online, or download an archive (prn_io.zip) that contains PRN_IO.C and PRN_IO.H. To use the module in your program, simply #include PRN_IO.H from wherever you need to call the functions, and compile and link PRN_IO.C into your program.

View The Source Code
Download The Source Code

The following tables list the details of how the software interfaces to hardware port. Refer to the source code itself for more information, or check out one of the links at the end of this page.

Printer Port Addresses

Printer Port Base Address

LPT1 0x0378 or 0x03BC

LPT2 0x0278 or 0x0378

LPT3 0x0278

Printer Port Registers

Register Name Address

Data Register Base + 0x00

Status Register Base + 0x01

Control Register Base + 0x02

Data Register Bit Definitions

Bit Function Low High

7 (MSB) D7 0 1

6 D6 0 1

5 D5 0 1

4 D4 0 1

3 D3 0 1

2 D2 0 1

1 D1 0 1

0 (LSB) D0 0 1

Status Register Bit Definitions

Bit Function Low High

7 (MSB) Busy Busy Not Busy

6 Acknowledge Nack Ack

5 Paper Status No Paper Paper

4 Selection Status Not Selected Selected

3 Error Status No Error Error

2 Not Used - -

1 Not Used - -

0 (LSB) Not Used - -

Control Register Bit Definitions

Bit Function Low High

7 (MSB) Not Used - -

6 Not Used - -

5 Not Used - -

4 Interrupt Control Interrupts Disabled Interrupts Enabled

3 Select Selected Not Selected

2 Initialize False True

1 Auto Feed True False

0 (LSB) Strobe (Active-Low) True False

Source Code Implementation Notes:

The code is written to use LPT1 at 0x0378; change it if you need to
The code doesn't attempt to use interrupts
The parallel port is also capable of enhanced modes (see the links below), but this code doesn't attempt to use them
This code assumes that the port is configured as a "standard" or "normal" port; configuring the port for EPP or ECP modes may or may not work

Other Parallel Port Information

Parallel Port Books:

Parallel Port Specs And Info:

Parallel Port Drivers:

IOPORT.OCX - Visual Basic Parallel Port Drivers
I/O from Win32 - Delphi code for the DriverLINX driver
Linux Parallel Port Home Page
Port95nt - Parallel Port Driver for WinNT
Port I/O - Controlling the parallel port from Visual Basic
Port I/O From BCB - Accessing the parallel port from C++ Builder and Delphi
TDLPortIO - A C++ Builder component for the DriverLINX Win95/98/NT Port I/O driver
TVicHW32 and TVicPort - Device drivers that allow Win32 hardware access
ZeeCube - Parallel Port Direct IO Driver for WinNT

Product Search

Home About This Site Add URL Contact Us Privacy Link To Us Search What's New?