Introduction To Parallel Port

What is a port?
A port contains a set of signal lines that the CPU sends or receives data with other components. We use ports to communicate via modem, printer, keyboard, mouse etc. In signaling, open signals are "1" and close signals are "0" so it is like binary system. A Standard Parallel Port sends 8 bits and receives 5 bits at a time. The serial port RS-232 sends and receive 1 bit at a time.

About Parallel Port
The Parallel Port is the easiest way to control devices outside the PC, like LEDs, lights and even home appliances. In fact the idea behind parallel port is really simple. It is an 8-bit parallel interface, so we have eight bits available there. Simple put, since each data bit can be set as either “0” (“turned off”) or “1” (“turned on”), we can directly turn on or off up to eight devices.

Parallel Port has mainly 3 modes of operation :-

1. SPP - Standard Parallel Port (Discussed in this article).
2. EPP - Enhanced Parallel Port (Not discussed in this article).For more info on EPP click here.
3. ECP - Extended Capabilities Port (Not discussed in this article).For more info on ECP click here.

How it looks?
On the PC the parallel port uses a 25-pin connector (called DB-25, 25-pin D-sub or 25-pin D-shell), as you can see in figure shown below.

Pin Configuration
The Parallel Port actually has 32 pins.

• 8 - Data Pins (shown in yellow)
• 8 - Control Pins (shown in red)
• 8 - Status Pins (shown in blue)
• 8 - Ground Pins (shown in green)
  

But only 25 pins were required to run a printer through it, so rest of the pins (not shown in above picture) were not made available outside the mother board.
So, we have in total 25 pins.

• 8 - Data Pins
• 4 - Control Pins
• 5 - Status Pins
• 8 - Ground Pins

Data Pins
Data pins are used to send 8-bits of data. Pins 2 to 9 are data pins or we can call them as an 8-bit data bus (D0 – D7). This data bus carries 8-bit data at any instant of time as shown below.

The type of data it carries is in digital form, i.e. either 0 or 1.

• ‘0’ means OFF or 0 volts.
• ‘1’ means ON or 5 volts.

For example, if pin 2 is carrying ‘0’, then it will give 0 volts at output. And if it is carrying ‘1’, then it will give 5 volts at its output. Similarly we get output as 0 volts or 5 volts from other data pins.

Now a question arises, how can we send data?

Answer to this is pretty simple. Every port on our computer has got some unique hexadecimal address. Parallel Port also has got a specific address. Generally this address is 0x378H.

To find out the address of the parallel port on your computer follow the steps written below:-

1. Open Device Manager.

2. Go to Ports (COM & LPT) item in it.

3. Double click ECP Printer Port (LPT1).
   

4. A new window will pop up.

5. In that window click Resources tab.

6. Now see the I/O Range. Mine is 0378–037F.

7. Here 0378 is the starting address of the Parallel Port.

Now we can send data using software programs (discussed later) to this address.

Now we know where to send data, but what data we have to send?

We have to send ‘0’ or ‘1’ to each pin (D0-D7) simultaneously.

Each Data Pin has been assigned a bit value as shown below.

Here, D7 is the most significant bit and D0 is the least significant bit.

This means, if we send any data say ‘16’ , then ‘1’ will be sent to D4 and ‘0’ to rest of the pins. Similarly ‘255’ will send ‘1’ to each pin.

We can also view it like this:-

• Binary representation of 16 is 00010000.

• Binary representation of 255 is 11111111.
  

See the table below for other decimal numbers.

So, every data is converted to 8-bit binary form and pins are send data accordingly.

Status Pins
These ports are made for reading signals. The range is like in data ports which are S0-S7. But S0, S1, S2 are invisible in the connector. And the address for this is 0x379.

Note- Address of data bus is same as that of parallel port. But address of Status Bus is 0x378 + 1, i.e. 0x379 (Address of Data Bus + 1).

Bit value for these is shown below.

Control Pins
This port usually used for sending as well as recieving data. The range is like in data ports C0-C7 but C4, C5, C6, C7 are invisible in connector. And the address for this is 0x37A.

Note- Address of Control Bus is 0x378 + 2, i.e. 0x37A (Address of Data Bus + 2).

Ground Pins
These pins are just used to complete the circuit attached to the parallel port.

In The End
This article was just an introduction to the parallel port.

Now you are ready to make your first parallel port controlled device. Read my next article on Interfacing LEDs using the Parallel Port.