Parallel Port Basics

Parallel port basics

What is a port?

On computer and telecommunication devices, a port is generally a specific place for being physically connected to some other device, usually with a socket and plug of some kind. Typically, and as an example, a personal computer is provided with serial ports, parallel ports, USB ports, fire wire ports, etc. The serial port for example supports sequential, one bit-at-a-time transmission to peripheral devices such as old scanners and the parallel port supports multiple-bit-at-a-time transmission to devices such as old printers. I'm keep saying old because USB replaced all this ports in the new devices such as scanners and printers, PDAs, game pads , joysticks, digital cameras and flash drives. USB port is relatively new and intended to replace many varieties of serial and parallel ports.

Why Ports are important?

I think you know the answer. We use ports to communicate to peripheral devices like modem, printer, keyboard, mouse, PDAs, digital cameras, personal media players, flash drives, and many others.

Parallel Port

A PC parallel port is a simple and inexpensive tool for building computer controlled devices and projects. The simplicity and ease of programming makes parallel port popular in electronics hobbyist world. The parallel port is often used in Computer controlled robots, Atmel/PIC programmers, home automation, etc...

Old Parallel Ports are used for output only where newer ones support bi-directional data transfer. Actually we prefer using the parallel port for output.

What defines the port to be input or output is the port mode of operation. There are 5 modes to choose from.I'am not going to explain more details about that as I recommend it for output only and in this case there is no need for any additional configurations , any configuration will work.

More about parallel port modes here:

http://www.bb-elec.com/tech_articles/parallel_port_explained.asp

Hardware
The Parallel Port Connector is a 25 pin female (DB25) and its pin outs is shown in the picture below

The pins in the parallel port connector are divided in to three groups, they are

1) Data lines (data bus)
2) Status lines
3) Control lines

As the names refer, data is transferred over data lines, Control lines are used to control the peripheral and the peripheral returns status signals back through the status lines.

These lines are actually connected to Data, Control and Status registers internally.

Parallel port registers

as we said; The Data, Control and status lines are connected to their corresponding registers inside the computer. Each register has its own address by which you can send data to or receive from. The Bios assign a base address (usually 0x378H or 888d) for this port and through it you can access the other registers as follows:

Data register is (base address + 0) = > e.g. 888

Status register is (base address + 1) => e.g. 889

Control register is (base address + 2) => e.g. 890

So by manipulating these registers in a program written by any language, one can easily send data to parallel.

How to know the address of the parallel port in your PC?

If you are running windows machine do the following

Go to Control panel > System > Hardware tab>device manger > Then select there the parallel port you are interested from Ports (COM & LPT) section.

Double click on it and choose resources tab you will find the addresses assigned to the parallel port. You will need these addresses when programming to send data for the parallel port. Typical command may be Output (888, intVal); or __outb(890, shortValue)

Note: This last screen shot may differ from PC to another according to the mode and type of the parallel port in your PC. The first address should be the base address (here 0x378) and you can increment it to access other registers.

The details of parallel port signal lines are given below

Pin No (DB25)	Signal name	Direction	Register - bit	Inverted
1	nStrobe	Out	Control-0	Yes
2	Data0	In/Out	Data-0	No
3	Data1	In/Out	Data-1	No
4	Data2	In/Out	Data-2	No
5	Data3	In/Out	Data-3	No
6	Data4	In/Out	Data-4	No
7	Data5	In/Out	Data-5	No
8	Data6	In/Out	Data-6	No
9	Data7	In/Out	Data-7	No
10	nAck	In	Status-6	No
11	Busy	In	Status-7	Yes
12	Paper-Out	In	Status-5	No
13	Select	In	Status-4	No
14	Linefeed	Out	Control-1	Yes
15	nError	In	Status-3	No
16	nInitialize	Out	Control-2	No
17	nSelect-Printer	Out	Control-3	Yes
18-25	Ground	-	-	-

NOTE:

Note the inverted pins in the control register and take care when sending values. If you need to send 1101 it will be 0100 because pin1,2 and 4 are inverted.

References

http://logix4u.net/Legacy_Ports/Parallel_Port/A_tutorial_on_Parallel_port_Interfacing.html

http://www.epanorama.net/circuits/parallel_output.html