Parallel Port Interfacing Part1

8/6/2019 Parallel Port Interfacing Part1

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PARALLEL PORT INTERFACING

BY:LAXMAN BHAIA


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Parallel port is same as the printer port at the back side of your computer. It looks like asshown below:

Interfacing through parallel port is very simple but very interesting also because of somemouth watering applications. But there are limitations in using the parallel port. And it is

the limited data rates up to 1Mbps. As the name suggests, it is called parallel port becausedata transfer through it take place parallelly rather than serially. You will find parallel

port at the back side of your CPU which looks like as shown above. Technically it isnamed as DB25 female connector. It consists in all 25 pins out of which there are 17 pins

for input and 12 pins for output and 8 pins are grounded. There are three registersinternally that control the printer or parallel port. These are:

1. Control register (4-bits)2. Status register (5-bits)3. Data register (8-bits)

We can input through all three registers and output through only data and control

registers. Each register have its own address and can be accessed using these addresses.First, I will explain the pin configuration of printer port. It is shown below:


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DATA PORT:

ADDRESS PIN NUMBER PIN NAME BIT NAME

Base + 0

(e.g. 0x378)

PIN 2 Data 0 0

PIN 3 Data 1 1

PIN 4 Data 2 2

PIN 5 Data 3 3

PIN 6 Data 4 4

PIN 7 Data 5 5

PIN 8 Data 6 6

PIN 9 Data 7 7

Base address : 0x378 or 0x278 (In hexadecimals)Generally it is 0x378. I will tell you the method of finding out this address.

Data output pins are driven by 74LS374 octal latch, which capable of sourcing 2.6mAcurrent and sink24mA current. There is 2.2nF capacitor between each pin and the ground

to reduce transients.Note : It is essential that any device does not try to pull the pins directly to ground. As

this will cause 74LS374 to source more current than it could handle.

S7

10111213 123456789

15 1416171819202122232425

D7 D6 D5 D4 D3 D2 D1 D0

S6 S5 S4 S3

C3 C2 C1 C0

Internal input/output

Data register

Control register

Status register

Control pins

Data pins

Status pins

Grounded pins

NOT of control NOT of status


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The input pins for the data registers are driven by 74LS244 tri-stated buffer. It isuninverted. The internal diagram of the parallel port is shown as follows:


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D0

D1

D2

D3

D4

D5

D6

D7

Q0

Q1

Q2

Q3

Q4

Q5

Q6

Q7

OUTPUT

FILP FLOP

74LS374

D0 (2)

D1 (3)

D2 (4)

D3 (5)

D4 (6)

D5 (7)

D6 (8)

D7 (9)

S

Y

ST

E

M

B

U

S E

E

INTERNAL DIAGRAM OF PARALLEL PORT

Q0

Q1

Q2

Q3

Q4

Q5

Q6

Q7

D0

D1

D2

D3

D4

D5

D6

D7

OUTPUTFILP FLOP

74LS244


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Now find out the address of the parallel port:

Step 1: Go to My Computer icon on your desktop and right click over it using your

mouse. Scroll down, go to properties and left click over it as shown below.


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Step 2: A window of system properties like shown below will appear. Now left click onthe hardware option.


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Step 3: A window like shown below will appear which shows three options, devicemanager, drivers and hardware profile. Now we have to left click on Device Manager.


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Step 4: now device manager window will open which shows all the devices that makes

your computer. You will see an icon like this is present in front of which

Ports (COM&LPT) is written. On clicking the respective options you can see all itsdetails. So, you have to just click on the plus sign.


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Step 5: As click on the plus sign, you will find some options according to yourcomputer as shown below:

Now double click on Printer Port (LPT1).


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Step 6: You will find Printer Port (LPT1) window as shown below. Click on theResources option.


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Step 7: Here is your parallel ports address.Note: 0378 is the address in hexadecimal and it the address of the data port of the parallel

port. Next three addresses i.e.; 0379, 037A and 037B are the addresses of the control port(037A) and status port (0379).

Once you find it click OK.


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So, far the things are going so simple. Now lets move on to programming part. We are

using Turbo C++ compiler to control our parallel port. Using this we can build

programs to send data to parallel port and take input data from parallel port using

commands outportb and inportb. Before moving ahead takes a small break andyes bring the multimeter as you come back from the break.If you dont have a multimeter then make a test device like the one shown below. this test

device is very simple and made of very little number of components which are easilyavailable everywhere i.e. some LEDs and a parallel port connector and some resistances.

10111213 123456789

15 1416171819202122232425

1K 1K 1K 1K 1K 1K 1K 1K


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Now you can attach this device to the parallel port. What you see is that all the LEDs areglowing.

You can see that the two LEDs in the middle are glowing at low intensity. This isbecause the LEDs that I used to make this circuit were not of very good quality. So theintensity may vary even if all the conditions like applied voltage and series resistance etc.

These LEDs shows that the voltage at all the pins is high. This voltage must be near to5V.

We should use multimeter to measure the voltages at the port pins. If you use multimeter,notice the reading of voltage of each pin. When you apply the red probe of multimeter to

data pins and black probe to grounds of parallel port while the multimeter is on dcvoltage measurement configuration, you will see a value at multimeter display.

You know what this value is, yes 5V and on all ports. Why because by default when it isdisabled by windows it shows high on all ports. If you try to read values of this port fromTC, then the value you get is 255, which is equivalent to high on each pin.

Parallel Port Interfacing Part1

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