1. بسم الله الرحمن الرحيم Table of contents:
The LPT Hardware.
Relationship Between (Digital Bit: 0 or 1) Binary & Decimal Systems
Outputting to a simple 8-LEDs display.
Output Circuit .
Controlling the LED display with Visual Basic.
Analog &Digital Input &Output.
Reading & Writing through (LPT).
Interface circuit Construction & description.
Controlling the interface sections.
Controlling the control Register.
References.
The common PC printer port has long been a convenient interface for the electronic hobbyist looking to control some external device with their computer
بسم الله الرحمن الرحيم
Controlled Systems Via Parallel Port
Controlled Systems Via Parallel Port

2. Parallel Port Hardware.
The standard Parallel port (also called Parallel Printer Port or LPT) has undergone a number of changes over the years. In the early 80’s the IBM standard printer port (SPP) provided only output data flow, from the computer to an peripheral device, usually the printer. It could not receive data from a peripheral device back into the computer except for a couple of status lines to tell the computer that the printer was out of paper, or off-line etc.
The original IBM-PC's Parallel Printer Port had a total of 12 digital outputs and 5 digital inputs accessed via 3 consecutive 8-bit ports in the processor's I/O space.

3. Parallel Port Hardware.
4 Output Pins (Three inverted) Accessed via the CONTROL Port.
25-way Female D-type Conector.
5 Input Pins (one inverted) Accessed via the STATUS Port
8 Output Pins Accessed via the DATA Port
Beside the data output port there is a five bit STATUS input port through which the computer could get status information from the printer such as paper out, or busy or printer online etc. These bits are read by reading the 5 most significant bits from the LPT1 base I/O address +1, usually 379 hex or 889 Dec
The DATA port contains eight bits of data that are accessible by writing to the LPT 1 (PC May contain two or three LPT ports) port base I/O address, usually 378 Hex or 888 Dec
Finally there is a four bit CONTROL port that is used to control various functions of the printer such as initializing, linefeed, etc. These bits can be controlled by writing to the least significant 4 bits of the LPT1 base I/O address +2, usually 380 hex or 890 Dec.

4. Parallel Port Hardware.
It’s very Significant to know Corresponding Decimal Values of each DATA Pins (D0 to D8) to Design nor Develop necessary software and external devices.
Decimal Values
The remaining 8 Pins – 18 through 25 – are Grounded.

5. Parallel Port (LPT) Hardware.
The bi-directional Printer Port
Although the original printer port specification (called SPP for Standard Parallel Port) did not allow for reading 8-bit data through the data port, this capability began appearing with IBM’s PS-2 line of computer systems. Most computer systems today that are sold with the serial and printer ports integrated into the motherboard contain bi-directional parallel ports.
On the other hand, many multi-function I/O cards that you can buy today to plug into your computers motherboard may still only contain the SPP parallel port specification. When choosing a parallel port or multi-function I/O card, ensure that the specification for the printer port is stated as being either EPP for Enhanced Parallel Port or ECP for Extended Capabilities Port. Either of these two specifications allow for bi-directional data flow.

6. Outputting to a simple 8-LEDs display.
Output Circuit:
To begin using Visual Basic to perform I/O tasks we need to construct a connection to the computers LPT port. We will use a DB-25 male connector which is connected via a 25-pin ribbon cable to a 24- pin DIP. In this first exercise using the LPT interface we will construct a simple circuit on the proto-board to see if we can simply output data using Visual Basic and have a group of eight LEDs respond to it.
Next Slide shows the circuit. Pins 2 through 9 are connected to the anode of each LED and the cathode of the LED is connected through a 470 to 1 Kohm resistor to any one of the internally connected ground pins 18 through 25. The resistors are used to limit the current from the LPT port through the LEDs.

7. Outputting to a simple 8-LEDs display.
Pins 2 through 9 are connected to the anode of each LED
and the cathode of the LED is connected through a 470 ohm to 1K resistor to any one of the internally connected ground pins 18 through 25. The resistors are used to limit the current from the LPT port through the LEDs.

8. Outputting to a simple 8-LEDs display
Circuit Operation:
The operation of this circuit is straight forward. Take note that this circuit requires no external power supply, the current required to illuminate the LEDs is delivered from the LPT port itself. Under software control data values are written to the data register of the LPT1 port by writing to I/O address 888. Valid data values range from 0 to 255(Decimal). When a data value is written the LEDs will illuminate. An illuminated LED reflects a digital “1” and a dark LED reflects a digital “0”.

9. Outputting to a simple 8-LEDs display
Controlling the I/O port with Visual Basic:
Unlike it’s predecessors such as Qbasic, Turbo-Basic or even the lowly GW-Basic, Visual Basic does not have the ability to directly access I/O ports on it’s own. We can, however add this capability to Visual Basic by using an OCX ( Object Control Extension).
There’s lots of technical language we could use to describe an OCX but basically it adds a control to the Visual Basic toolbox that can be placed on a form which will allow us to perform input and output operations from any I/O port including the LPT port.
It is available as a shareware program on the Internet.

10. Outputting to a simple 8-LEDs display
LPT Output Test Program
Entering a value into the text box at any time and then pressing ENTER (or Clicking Display button ) will cause the entered value to be sent to the LPT port. The value entered must be valid for an 8-bit value. This is from 0 to 255.
Any value entered outside this range will cause an error message to be displayed and the text box will be cleared.
So, Present Decimal Value [64] (in the Text box), will turn LED No. 7 (D6 = 1) On .

11. Reading & Writing through the Printer Parallel Port.
we will take the interfacing of the printer port one step further by building a circuit that will allow digital and analog input and output from a single LPT port.
This project has two parts;
constructing the Input/Output circuit .
writing the test routine.

12. Reading & Writing through the Parallel Port
Interface Circuit Description
. It contains the following sections:
Power Supply
The circuit is powered by a single 5 volt supply which can be constructed using a 9volt battery and the 7805 voltage regulator.
An alternate arrangement is to use a 7vDC or higher wall adapter and connect it into the circuit as you would the battery.
The 7805 regulator can regulate any supply voltage from 7 to 25v DC down to the required 5 v for this circuit.
Building the Interface……...
Simple Printer Port Interface Circuit

13. A 74LS245 octal bi-directional latch
A 74ALS574 octal D Latch
to provide an 8-bit digital output port which will be used to light 8 LEDs or any device that you wish to send digital signals to.
A 74LS245 octal bi-directional latch
to provide an 8-bit digital input port which will be connected to 8 switches or any devices that you wish to read digital information from.
An Analog Devices AD558 8-bit digital to analog converter
to provide an analog output ranging from 0 to 2.55 volts. The AD 558 is unique in that it has an on-board 8-bit latch that we will use to hold the data that is delivered to it from the printer port.
Analog Output
0 to 2.55 Volt
A National Semiconductor ADC bit analog to digital converter.
That will read an analog input from 0 to 5 volts and provide an 8-bit corresponding output to the printer port. Initially, the analog input will be connected to a 100K to 500K(or any Sensor ) potentiometer that is connected between +5volts and ground to provide an analog input signal.
Simple Printer Port Interface Circuit
Analog Input
0 to 5 Volts

14. Reading & Writing through the Parallel Port
Interface Circuit Description
Each IC will be controlled by one of the bits of the CONTROL section of the LPT port ( indicated by blue circle in previous slide ). These control bits will be manipulated by Visual Basic to latch data to be written out of the interface or to read data from the digital or analog inputs to the computer.
The 8 data lines of the LPT port now form a data bus, which at times will contain data to be read by the computer and at other times will carry the data to be delivered from the computer to the digital or analog output.
Constructing the Interface
Use the circuit shown to build the complete printer port interface. The layout of the parts should occupy 3 proto-boards (Proto-boards are arranged to accept integrated circuit chips and electronic components and allows the interconnection of the components to form complete circuits. ), one for the DIP-24 connector from the LPT port cable, one to hold all four IC’s in a row as illustrated in the circuit diagram and a final one to contain the 8-bit DIP switch, the 8 LEDs and the potentiometer. Take the time to lay out a neat circuit. This will always make trouble-shooting easier if it is necessary later. Note: The jumper wire from the ADC0804 pins #3 and #5 to ground is usually left open. Normally this circuit will trigger and begin performing free-running A/D conversions at power-up. If it does not, then touching this connection to ground momentarily will trigger the circuit.

15. Reading & Writing through the Parallel Port
Controlling the Interface Sections
In short, we need to have control in our hardware and our software to be able to determine what each section of the interface will do and when it is allowed to do it. The hardware control is achieved through the use of four control lines from the LPT port. These lines are C0, C1, C2 and C3 which are connected to LPT pins 1, 14, 16 and 17 respectively. Each time we wish to cause a value to be read or written to a device in our interface we must change the state of it’s control line. This will allow the device to either latch the data on the data bus if it is an output device or to place data onto the data bus if it is an input device.
Controlling the Interface Sections
Using multiple devices all connected to a common data bus raises some design and programming issues.
Control of the data bus
One and only one device may place data on the data bus at a time. If we wish to send digital or analog signals out of the computer, the computer will have control of the data bus. If we wish to read digital or analog information into the computer, the digital input IC or the analog to digital converter will have control of the data bus. It becomes the job of the program to make sure that only one of the interface sections will have access to the data bus at a time in order to read data from or to place data onto the bus.
Selecting Input or Output devices
Suppose we wish to output a digital value to the LEDs and so we therefore cause the computer to place a digital value onto the data bus. We want the digital output latch, the to then react to the data on the bus and deliver it to the LEDs. We must ensure that the digital to analog converter, the ADC0804 does not react to the digital value that is placed on the data bus.

16. Reading & Writing through the Parallel Port
Controlling the Control Register
In this project we need to control four bits of the control register. For most systems with a single printer port, designated as LPT-1 this would be at address (or called Base address) 888 (dec) or 378 (hex).
If there was a second printer port; LPT-2 it’s address would be 632 (dec) or 278 (hex). As we have seen before, each LPT port has at least 3 sections. Each section has it’s own register, or I/O address.
The data register is at the base address of the LPT port
The Status register is at the base address +1 of the LPT port
The Control register is at the base address+2 of the LPT port
In order to control the bits of the control register we must write to the base address + 2 of the LPT port we are connected through.
((Notice)) that control lines C0, C1 and C3 are inverted logic. This means that when a “1” is written to that bit by the program, the logic level at the respective pin at the LPT port is “0”. Only C2, the third bit at pin #16, which controls the AD558 digital to analog converter is positive logic. It must be kept high most of the time until it is brought low to trigger the AD558 to read the data bus and deliver a corresponding analog output.

18. References References Electronic Devices [Floyd].
Electronic Circuit [Shaum].
Upper Canada Technologies Online [
Electronics Information Online:, [
Hyper Physics [
E4All [
IC's Conf. Online [
MS-V Basic 5 [Abu El-Ata M. M.].
References