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Interfacing the LCD module to Parallel Port

(3rd article in Parallel port programming series)    by  Harsha Perla

                 You have seen LCD modules used in many of the electronics devices like coin phone, billing machine and weighing machines. It is a powerful display options for stand alone systems. Because of low power dissipation, high readability, flexibility for programmers; LCD modules are becoming popular. In this article, we will learn how to connect LCD module to PC parallel port and we will prepare some library routines for LCD interfacing.



Before starting our study, let us know why you need to interface LCD or Liquid Crystal Display module to the parallel port .

  • We can easily program the parallel port. (To know parallel port programming, we have these two articles in electroSofts.com:   Parallel Port Programming (PART 1) : with C, in DOS
                                          Parallel Port Programming (PART 2): with VC++, in Widows
    If you don't know how to program the parallel port, you must read the first part of the articles mentioned above. So, to study the LCD module programming, this is easiest method I think.)

  • If you need to modify the code, you need not have to disconnect the circuit or re-program the chip as you do in the case of microcontroller.

  • You need to spend less: One LCD module, D25 female connector, one potentiometer (optional), and some wires- this is what you need along with a computer.

  • When you are using computer in full screen mode like Games, movie or TV; You need to exit the application to get a small updating information from the computer. i.e., if you need to watch time in that time, you have to close the games. But instead of that you can use LCD module to display real time from the PC and you can use it along with your application. Real time  implementation from the system clock example is explained in this article. If you are good in programming, even you can connect to the internet to get news, stock exchange updates and make them flash in the LCD module, only if you found it important, you can go through it by exiting your application.

LCD modules are available in a wide range like 8x1, 8x2, 16x1, 16x2, 20x2, 20x4, 40x4. Here we have used 16x2- that means 2 rows of 16 characters. It is a Hitachi HD44780 compatible module, having 16 pins including 2 pins for backlight.

 Following table gives pin structure of LCD module. LCD modules without backlight will have only 14 pins. If you are using such LCDs, simply ignore 15th and 16th pins.


Pin No Symbol Details
1 GND Ground
2 Vcc Supply Voltage +5V
3 Vo Contrast adjustment
4 RS 0->Control input,    1-> Data input
5 R/W Read/ Write
6 E Enable
7 to 14 D0 to D7 Data
15 VB1 Backlight +5V
16 VB0 Backlight ground

To program the LCD module, first we have to initialize the LCD by sending some control words. RS should be low and E should be high when we send control. R/W pin 0 mean write data or control to LCD and R/W pin 1 means read data from the LCD. To send a data to LCD, make RS high, R/W low, place the data in pins 7 to 14 and make pin E high and low once. You can understand exact method after seeing the code, later in this tutorial. To make this let us first build a circuit.

Here, we are going to write on the LCD module and not reading back. So, R/W is connected to ground directly. We need not have to input any data through, so all output pins are used in our application. Data pins of LCD are connected to data pins of the port. Strobe signal (Pin 1 of D25 connector) is given to E (Pin 6 of LCD), Select printer (Pin 17 of D25) is connected to RS (pin 4 of the LCD).

In the above diagram, LCD module is connected to the lpt port using D25 male connector. Pin number 3 of the LCD is for adjusting the contrast, connected in such a way that it can be varied from 0V to 5V. Keep it to 0 initially.

If everything is OK, you should get the LCD module as follows when power is switched ON.

If you get this screen, then we can start programming. Otherwise check your connections, try by varying the 10K potentiometer. If you get this display also, you can get maximum clearness by varying the pot. For me, pot was needed to be nearly 0V. So, it is OK if you don't use pot also, just connect pin 3 to ground.

Following table explains how to write control words. When RS=0 and R/W=0, data in the pins D0 to D7 will have following meaning.

Instruction D7 D6 D5 D4 D3 D2 D1 D0 Description
Clear Display 0 0 0 0 0 0 0 1 Clears Display and returns cursor to home position.
Cursor home 0 0 0 0 0 0 1 X Returns cursor to home position. Also returns display being shifted to the original position.
Entry mode set 0 0 0 0 0 1 I/D S I/D = 0 --> cursor is in decrement position.       I/D = 1 --> cursor is in increment position.
S = 0 --> Shift is invisible.      S = 1 --> Shift is visible
Display ON- OFF Control 0 0 0 0 1 D C B D- Display, C- cursor, B- Blinking cursor
 = 0 --> OFF
 =1 --> ON
Cursor/ Display Shift 0 0 0 1 S/C R/L X X S/C = 0 --> Move cursor.     S/C = 1 --> Shift display.
R/L = 0 --> Shift left.     R/L = 1 --> Shift right
Function Set 0 0 1 DL N F X X DL = 0 --> 4 bit interface. DL = 1 --> 8 bit interface. N = 0 --> 1/8 or 1/11 Duty (1 line). N = 1 --> 1/16 Duty (2 lines). F = 0 --> 5x7 dots.     F = 1 --> 5x10 dots.

I have left other instruction related to read and write LCD RAM area, we will see them later. Using these information, we will write some routines for basic functions of LCD. Now look at our first program below. Here I have written functions for all our needs in LCD interfacing. So, in our next programs, we are going to change our "main" function only.  You can save these function as library and include in your next programs if you want.

#include <dos.h>
#include <string.h>
#include <conio.h>
#include <time.h>

#define PORTADDRESS 0x378 /* Enter Your Port Address Here */


void lcd_init(void);
void lcd_write(char char2write);
void lcd_putch(char char2write);
void lcd_puts(char * str2write);
void lcd_goto(int row, int column);
void lcd_clear(void);
void lcd_home(void);
void lcd_cursor(int cursor);
void lcd_entry_mode(int mode);

void main(void)

lcd_puts("Welcome To");

while(!kbhit() ) //wait until a key is pressed...


void lcd_init()

outportb(CONTROL, inportb(CONTROL) & 0xDF);
//config data pins as output

outportb(CONTROL, inportb(CONTROL) | 0x08);
//RS is made high: control (register select)

lcd_write( 0x01);
lcd_write( 0x38);


void lcd_write(char char2write)

outportb(DATA, char2write);
outportb(CONTROL,inportb(CONTROL) | 0x01); /* Set Strobe */
outportb(CONTROL,inportb(CONTROL) & 0xFE); /* Reset Strobe */


void lcd_putch(char char2write)

outportb(CONTROL, inportb(CONTROL) & 0xF7);
//RS=low: data


void lcd_puts(char *str2write)

outportb(CONTROL, inportb(CONTROL) & 0xF7);
//RS=low: data


void lcd_goto(int row, int column)

outportb(CONTROL, inportb(CONTROL) | 0x08);
if(row==2) column+=0x40;
/* Add these if you are using LCD module with 4 columns
if(row==2) column+=0x14;
if(row==3) column+=0x54;

lcd_write(0x80 | column);


void lcd_clear()

outportb(CONTROL, inportb(CONTROL) | 0x08);


void lcd_home()

outportb(CONTROL, inportb(CONTROL) | 0x08);


void lcd_entry_mode(int mode)

if you dont call this function, entry mode sets to 2 by default.
mode: 0 - cursor left shift, no text shift
1 - no cursor shift, text right shift
2 - cursor right shift, no text shift
3 - no cursor shift, text left shift

outportb(CONTROL, inportb(CONTROL) | 0x08);
lcd_write(0x04 + (mode%4));


void lcd_cursor(int cursor)

set cursor: 0 - no cursor, no blink
1 - only blink, no cursor
2 - only cursor, no blink
3 - both cursor and blink

outportb( CONTROL, inportb(CONTROL) | 0x08 );
lcd_write( 0x0c + (cursor%4));


Output of this program should be like this:

I need not give details to all the functions above. You can understand them yourself. So, try using all the functions. In the next examples, we will generate a program that displays the system time in the LCD module. It may not be having much use in DOS, but if you transfer the same to Windows, you will gain many benefits. Also, if your computer will be working in DOS most of the time, you can think of writing an TSR for the same.

Program to display date and time in an LCD module: Just replace the 'main' of previous program with the following and run.

void main(void)

struct time t;
struct date d;
char strtime[17];

cputs(" ");
cputs(" ");



sprintf(strtime,"%02d:%02d:%02d", t.ti_hour%12, t.ti_min, t.ti_sec);

sprintf(strtime,"%02d:%02d:%4d", d.da_day, d.da_mon, d.da_year);



I will stop this article here itself. You can change these program if you want to interface LCD with microcontroller. We have many other interesting articles on port programming in www.electrosofts.com and other websites. Click here get a list of interfacing related articles.

Use Interfacing section of electrosofts Forum for your comments and doubts. I am getting many emails and I may not be able to answer all of them.


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