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Parallel Port Programming (PART 1): with C

By HarshaPerla

Parallel port is a very commonly known port, widely used to connect the printer to the PC. If you see backside of your computer, there will be a port having 25 pins with a small symbol like this:. That port is known as LPT port or printer port. We can program this port for device control and data transfer. In this article, we will learn basics of parallel port and programming the parallel port.

Parallel port basics:

In computers, ports are used mainly for two reasons: Device control and communication. We can program PC's Parallel ports for both. Parallel ports are mainly meant for connecting the printer to the PC. But we can program this port for many more applications beyond that.

Parallel ports are easy to program and faster compared to the serial ports. But main disadvantage is it needs more number of transmission lines. Because of this reason parallel ports are not used in long distance communications. Let us know the basic difference between working of parallel port and serial port. In serial ports, there will be two data lines: One transmission and one receive line. To send a data in serial port, it has to be sent one bit after another with some extra bits like start bit, stop bit and parity bit to detect errors. But in parallel port, all the 8 bits of a byte will be sent to the port at a time and a indication will be sent in another line. There will be some data lines, some control and some handshaking lines in parallel port. If three bytes of data 01000101 10011100 10110011 is to be sent to the port, following figures will explain how they are sent to the serial and parallel ports respectively. We can understand why parallel port communication is faster compared to that of serial.

Parallel port basics:

In computers, ports are used mainly for two reasons: Device control and communication. We can program PC's Parallel ports for both. Parallel ports are mainly meant for connecting the printer to the PC. But we can program this port for many more applications beyond that.

Parallel ports are easy to program and faster compared to the serial ports. But main disadvantage is it needs more number of transmission lines. Because of this reason parallel ports are not used in long distance communications. Let us know the basic difference between working of parallel port and serial port. In serial ports, there will be two data lines: One transmission and one receive line. To send a data in serial port, it has to be sent one bit after another with some extra bits like start bit, stop bit and parity bit to detect errors. But in parallel port, all the 8 bits of a byte will be sent to the port at a time and a indication will be sent in another line. There will be some data lines, some control and some handshaking lines in parallel port. If three bytes of data 01000101 10011100 10110011 is to be sent to the port, following figures will explain how they are sent to the serial and parallel ports respectively. We can understand why parallel port communication is faster compared to that of serial.

In the PC there will be D-25 type of female connector having 25 pins and in the printer, there will be a 36-pin Centronics connector. Connecting cable will combine these connecter using following convention. Pin structure of D-25 and Centronics connecters are explained bellow.  

figure 1.2	D25- Pin Number	Centronics 36 Pin Number	Function
	1	1	Strobe
	2 to 9	2 to 9	Data Lines
	10	10	Acknowledgement
	11	11	Busy
	12	12	Out of Paper
	13	13	Select
	14	14	Auto feed
	15	15, 32	Error
	16	16, 31	Init
	17	17, 36	Select In
	18 to 25	18 to 30, 33	GND
	-	34, 35	N/C

Table 1.0: Pin numbers and functions

 Now let us know how communication between PC and printer takes place. Computer places the data in the data pins, then it makes the strobe low. When strobe goes low, printer understands that there is a valid data in data pins. Other pins are used to send controls to the printer and get status of the printer, you can understand them by the names assigned to the pins.

To use the printer port for applications other than printing, We need to know how ports are organized. There are three registers associated with LPT port: Data register, Control register and Status register. Data register will hold the data of the data pins of the port. That means, if we store a byte of data to the data register, that data will be sent to the data pins of the port. Similarly control and status registers. The following table explains how these registers are associated with ports.

             Table 1.1: Pin directions and associated registers.

* Pins with * symbol in this table are hardware inverted. Than means, If a pin has a 'low' ie. 0V, Corresponding bit in the register will have value 1.

Signals with prefix 'n' are active low. That means, Normally these pins will have low value. When it needs to send some indication, it will become high. For example, Normally nStrobe will be high, when the data is placed in the port, computer makes that pin low.

Normally, data, control and status registers will have following addresses. We need these addresses in programming later.

Note: All the parallel ports do not have bidirectional capability. Earlier parallel ports had only output enabled in data pins since printers only inputs data. But latter, to make parallel port capable of communicating with other devises, bidirectional ports are introduced.

By default, data port is output port. To enable the bidirectional property of the port, we need to set the bit 5 of control register.

 To know the details of parallel ports available in your computer, follow this procedure:

• Right click on My Computer, go to "Properties".

• Select the tab Hardware, Click Device manager.

• You will get a tree structure of devices; In that Expand "Ports(Com1 & LPT)".  

• Double Click on the ECP Printer Port(LPT1) or any other LPT port if available.

• You will get details of LPT port. Make sure that "Use this Port (enable)" is selected.

• Select tab recourses. In that you will get the address range of port.

To start programming, you will need a D-25 type Male connector. Its pin structures can be found in the connector as follows:

Programming the printer port in DOS:

To start programming the port, we will use DOS. In DOS we have commands to access the port directly. But, these programs will not work on the systems based on Windows XP, Windows NT or higher versions. For security reason, higher versions of the windows does not allow accessing the port directly. To program the parallel port in these systems, we need to write kernel mode driver. In the part II, I am going to explain about programming the parallel port in windows XP. If you want to run the same program in Windows XP, For studying you can use the technique that I have posted in this forum.

When we want to find out whether particular pin of the port is high or low, we need to input the value of corresponding register as a byte. In that, we have to find out whether the corresponding bit is high or low using bitwise operators.  We can't access the pins individually. So, you need to know basic bitwise operations.

Main bitwise operators that we need are bitwise AND '&' and bitwise OR '|'. To make a particular bit in a byte high without affecting other bits, write a byte with corresponding bit 1 and all other bits 0; OR it with original byte. Similarly, to make particular bit low, write a byte with corresponding bit 0 and all other bits 1; AND it with original byte.

In Turbo C, there are following functions used for accessing the port:

• outportb( PORTID, data);

• data = inportb( PORTID);

• outport( PORTID, data);

• data = inport( PORTID);

outport() function sends a word to port, inport() reads a word from the port. outportb() sends a byte to port and inportb() reads a byte from the port. If you include DOS.H header, these functions will be considured as macro, otherwise as functions. Function inport() will return a word having lower byte as data at PORTID and higher byte as data at PORTID+2. So, we can use this function to read status and control registers together. inportb() function returns byte at PORTID. outport() writes the lower byte to PORTID and higher byte to PORTID+1. So this can be used to write data and control together. outportb() function write the data to PORTID. outport() and outportb() returns nothing.

Let us start with inputting first. Here is an example program, copy it and run in Turbo C or Borland C without anything connected to parallel port. Then you should see data available in status register and pin numbers 10, 11, 12, 13 and 15 of the parallel port. Pin 11 (active low) is 0 and all other pins are 1 means it is OK.

To understand bitwise operations: you want to find data in pin 15, value of (data & 0x08) will be 0x08 if bit 3 of register is high, 0therwise.   

We will use the same logic throughout the article.

Now, take a D-25 male with cables connected to each pins. Short all the pins from 18 to 25, call it as ground. Now you can run above program and see the change by shorting pins 10, 11, 12, 13 and 15 to ground. I prefer using switches between each input pins and ground. Be careful, do not try to ground the output pins.  

To find out the availability of ports in a computer programmatically, we will use the memory location where the address of port is stored.

If you run the the following code in Turbo C or Borland C, You will get the addresses of available ports.

         Next we will go to check output pins. To check the output, we will use LED's. I have driven LED's directly from the port. But it is preferred to connect a buffer to prevent excessive draw of current from the port. Connect an LED in series with a resister of  1KW or 2.2KW between any of the data pins(2 to 9) and ground. With that, if you run the program given below, you should see the LED blinking with app. 1 sec frequency.

I will stop this part here itself.  Next part of this article is now ready. In the PART 2, you will learn programming the parallel port in VC++. PART 2 is designed for the beginners of VC++. Click here to Read part 2.  

Part 3 is having the example using LCD module. There we are going to learn connecting LCD module to parallel port. Read part 3.

Also Read...

-Programming the Parallel Port(PART 2): with VC++
-Programming the 16x2 LCD module with Parallel Port: Example 1
-PC Based Game show/Quiz buzzer: Interfacing project example 2
-Serial communication via RS232 with C
-Links to port programming related articles in other sites