Computer Systems 2: Hardware Interfacing 1. Introduction

COMP1208 COMPUTER SYSTEMS 2008 

1. Introduction 

Computer Systems 2: Hardware Interfacing 

In this chapter we will look at how we can connect our computer to hardware, so we can read in 

information from the hardware and send out control signals to the hardware. The aim of this chapter is to 

explore how C++ programs can be used to interface simple data sources to a microcomputer 

1.1 What is a data acquisition system 

“A system that quantifies and stores data.” 

Figure 1: Data Acquisition System 

Physical Phenomena 

Data acquisition systems need to get real-world signals into the computer. These signals come from a 

diverse range of instruments and sensors, and each type of signal needs special consideration. 

Sensors and Actuators (transducers) 

A transducer is a device that converts input energy of one form into output energy of another form. A 

sensor is a device that receives a signal or stimulus and responds with an electrical signal. An actuator is 

the device that converts an electrical signal into a physical signal or stimulus of some type. Sensors and 

actuators can both be transducers. For example, a microphone is a sensor that converts sound energy (in 

the form of pressure) into electrical energy, while a loudspeaker is an actuator that converts electrical 

energy into sound energy. 

Signal conditioning hardware 

Sensor signals are often incompatible with data acquisition hardware. To overcome this incompatibility, 

the signal must be conditioned. For example, you may need to condition an input signal by amplifying it 

or by removing unwanted frequency components. Output signals may need conditioning as well. 

Data Acquisition Hardware 

At the heart of any data acquisition system lies the data acquisition hardware. The main function of this 

hardware is to convert analogue signals to digital signals, and to convert digital signals to analogue 

signals. 

Notes02-Interfacing.doc RevA Page 1-12 13/02/2008 14:11


The computer 

The computer provides a processor, a system clock, a bus to transfer data, and memory and disk space to 

store data. 

Software 

Data acquisition software allows you to exchange information between the computer and the hardware. 

For example, typical software allows you to configure the sampling rate of your board, and acquire a 

predefined amount of data. 

1.2 Example of System 

In this course we will design a simple system to control temperature on a circuit board. 

MOTOR 

HEATER 

OUT 

PORT 

Computer 

TEMP 

sensor 

Analogue to 

Digital 

Converter 

(ADC) 

IN 

PORT 

Figure 2: Example – Temperature Control System 

A temperature sensor is connected to the computer via a Analogue to digital converter. The computer 

continuously monitors the temperature. If the temperature is too high, a signal is sent to the motor to turn 

on a fan until the temperature decreases. If the temperature is too low, a signal is sent by the computer to 

the heater to turn it on until the temperature rises to the required level. 



1.3 Analogue and Digital Signals 

Analogue signals are continuous electrical signals that vary in time as shown the figure below. An 

analogue signal is continuously variable and has has an infinite number of values between some 

maximum and minimum. 

25 

Temperature 

Degrees C 

20 

9 12 3 6 

Hour 

Figure 3: Analogue Signal Example 

Analogue signals represent some physical quantity, i.e. they are a ‘MODEL’ of the real quantity. 

Digital signals are non-continuous, they change in individual steps. They consist of pulses or digits with 

discrete levels or values. The value of each pulse is constant, but there is an abrupt change from one digit 

to the next. A binary signal can only have one of two levels, logic 1 or 0. For example:5 Volts -> Logic 1 

and 0 Volts -> Logic 0 

Voltage 

5 

0 

Time 

0 1 1 0 0 1 1 0 .. 

Figure 4: Digital Signal Example 

Here, then, is a summary of the advantages and disadvantages of the two systems: 

Analogue 

Advantages 

Produces a more 'faithful' reproduction of the physical 

quantity. 

Usually simple 

Disadvantages 

Noise and distortion problems 

Digital 

Advantages 

Can be very immune to noise 

Signal can be transmitted over long distances. 

Disadvantages 

Output subject to quantity errors from sampling 

Can be complex 



1.4 Serial vs Digital communications 

When we wish to connect our computer to external hardware, there are two basic types of communication 

ports we can use: serial and parallel. 

Serial Communications 

source 

1 

0 

1 

0 

1 

1 

0 

0 

1 

0 1 0 1 1 0 0 

Destination 

Parallel Communications 

source 

1 

0 

1 

0 

1 

1 

0 

0 

1 

0 

1 

0 

1 

1 

0 

0 

Destination 

Figure 5: Analogue Signal Example 

Serial Communications: 

When information is sent across a single path, one data bit at a time, this is called serial communications. 

There may be number of serial communications ports on your PC, for example the 'D' shaped 9-pin 

connector on the back of the computer. This is a serial connector typically uses 2 loops of wire (1 in each 

direction) for data communication, plus additional wires to control the flow of information. However, in 

any given direction, data is still flowing over a single wire. 

Parallel Communications 

When information is sent over more wires simultaneously, many bits at a time, this is called parallel 

communication. For example, the 25-pin parallel port on your PC has eight data-carrying wires so that 

eight bits can be sent simultaneously. Because there are 8 wires to carry the data, the data finishes being 

transferred eight times faster than a serial connection. 



2. Interfacing the Parallel Port 

2.1 Introduction 

Communications between a computer and an external device require a medium, a common language (a 

code, such as ASCII code), and rules for the exchange of signals - called a protocol. The protocol 

includes such things as rate at which the exchange of data can take place and how to control the flow of 

data when necessary. 

This section gives an introduction to interfacing the parallel port. 

2.2 Ports 

A port is an eight-bit register. It can be accessed by software, but it is also physically connected to a 

hardware device, or more usually, it is part of a hardware device. A PC can have 65,536 ports connected 

to it, numbered from 0 to FFFF in hex. However, only a small number of these addresses are actually 

used. 

2.2.1 Common DOS functions in C/C++: 

To access a port using the C/C++ language, MicroSoft Visual C++ provides the following functions, 

which it should be noted do not conform to ANSI C++ standards: 

_inp() 

reads a byte from a specified port. 

_outp() writes a byte to a specified hardware port. 

• _inp(portno) reads a byte from the port with the address portno. 

The variable portno must be an unsigned integer between 0 and FFFF hex, and the function returns an integer 

value. 

• _outp(portno,value) writes a byte to the port with the address portno. 

This causes the information in value to be written to the port with the address portno. Again portno must be an 

unsigned integer as before, and value is also an integer. 

Note these functions will only work if running from DOS or Windows 98, if you are working on 

Windows NT/2K/XP, you will need the following .dll loaded: inpout32.dll and inpout32.lib. 

How to use inpout32 in VC++ 6.0 application 

1 Login as administrator 

2 Copy inpout32.dll to your Windows system directory (e.g \Windows\System32), 

2 Add the header information below to your VC++ code. 

3 Add inpout32.lib file to your project and build. 

The following header information is required to map the _inp and _outp functions to Inp32 and Out32 

respectively: 

// inpout32.dll function prototypes 

short _stdcall Inp32(short port); 

void _stdcall Out32(short port, short data); 

// For compatibility with Win9x code 

#define _inp Inp32 

#define _outp Out32 



3. Parallel Port 

3.1 Interfacing the Parallel Printer Port: 

A PC can be connected to a parallel printer by means of a 25-pin connector as shown in 

Figure 6 below. 

ref 

Figure 6: 25-way Female D-Type Connector 

There are three ports associated with the computer printer interface as shown. 

• DATA Port - access to 8 output pins 

• STATUS Port - access to 5 input pins (one inverted) 

• CONTROL Port - access to 4 input/output pins (three inverted) 

• the remaining 8 pins are grounded 

The data port is used to output the printable characters to the printer. The status port is used to input the 

information from the printer so that the computer will know when and if to send information to it. The 

control port, is used to output control information to the printer. 

When a computer is first turned on, the BIOS (Basic Input/Output System) will determine the number of 

ports that exist and assign device labels LPT1, LPT2 and LPT3 to them. LPT1 is normally assigned base 

address 378h, while LPT2 is assigned 278h. However this may not always be the case. 

Address Ports Address Ports 

0x378 Data Port for primary parallel port (LPT1) 

0x379 Status Port for primary parallel port (LPT1) 

0x278 Data Port for second parallel port (LPT2) 

0x279 Status Port for second parallel port (LPT2) 

0x37A Control Port for primary parallel port (LPT1) 0x27A Control Port for second parallel port (LPT2) 

Table 1: Port Addresses 



3.1.1 Software Registers for Standard Parallel Port 

Data Port 

The base address, usually called the Data Port or Data register is simply used for outputting data on the 

Parallel Port’s data lines (pin 2-9). This register is normally a write only port. If you read from the port, 

you should get the last byte sent. However, if your port is bi-directional, it is possible to receive data. 

The Data Port has the Port Address 0x378, the 0x indicating a hexadecimal number. The Data Port is an 

output port when used by the printer, but it can be used by other applications as an input port. 

Data register 

BIT7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0 

 

Pin 9 Pin 8 Pin 7 Pin 6 Pin 5 Pin 4 Pin 3 Pin 2 

Figure 7: 

Data Port Assignment 

Control Port 

The Control Port has the Port Address 0x37A and was intended as a write only port. With a printer 

attached to the Parallel port, four controls are used, Strobe, Auto LineFeed, Initialise and select printer. 

However, it is possible to use these lines for inputs also. 

Bit 4 and 5 are internal controls. Bit 4 will enable the IRQ and bit 5 will enable the bi-directional port, 

meaning you can input 8 bits using the data registers. 

unused unused Enable 

BiDir 

Enable 

IRQ 

____ 

Select 

Printer 

Initialise 

Printer 

____ 

Auto 

Linefeed 

____ 

Strobe 

BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0 

 

Pin 17 Pin 16 Pin 14 Pin 1 

Figure 8: 

Control Port Assignment 

Status Port 

The third port is the Status Port and it has the Port Address 0x379 and is a read only port. Any data 

written to this port will be ignored. The Status port is made up of 5 input lines 

(Pin 10,11,12,13,15), a IRQ status register and two reserved bits. 

Busy 

____ 

ACK 

Paper 

Out 

Select 

In 

____ 

Error 

___ 

IRQ 

(Not) 

Reserved 

Reserved 

BIT7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0 

› › › › › 

Pin 11 Pin 10 Pin 12 Pin 13 Pin 15 

Figure 9: 

Status Port Assignment 



3.2 VC++ Examples 

3.2.1 Input Data from Port 

Consider the following example, which sets up the parallel port to read from the data port. 

Example: Port_in.cpp 

//Program to initialise parallel port to read from an Application Card 

#include 

#include 

using namespace std; 

int main() 

{ 

int data; 

_outp(0x37A_PORT,0x20); 

data = _inp(0x378); 

// set bit 5 on Control Reg -> read data 

// byte read from the Data Port 

cout


This above program is modified to include a function which handles the read from parallel port and also 

the header is modified to include the function prototypes for the inpout32.dll required use on Windows 

NT/2000/XP: 

//Program to initialise parallel port to read from an Application Card 

// C/C++ Library includes 

#include 

#include 


// inpout32.dll function prototypes required for Windows NT/2000/XP 






// Parallel Port Addresses 

#define CNTRL_PORT 0x37A 

#define DATA_PORT 0x378 

// Printer control port location 

// Printer data port location 

// Function Prototype 

int read_pport( void ); 

int main() 

{ 

int data; 

data = read_pport(); 

//call function to read from the Data Port 

cout


3.2.2 Output Data to Port 

Consider the following example, which sets up the parallel port to write to the data port. 

Example: Port_out.cpp 

// Program to initialise parallel port to write to an Application Card 

// IDE: Microsoft VC 6.0 


#include 

#include 





int main() 

{ 

_outp(CNTRL_PORT,0x01); 

_outp(DATA_PORT, 0xA7); 

return 0; 

} 



// set bit 0 on Control Reg -> write data 

// write 0xA7 to parallel port 

Using the _outp function, the control port is set to 0x01, i.e. 0000 0001 2. 


unused unused Enable Enable IRQ Select Initialise Auto Strobe 

BiDir 

Printer Printer Linefeed 

0 0 0 0 0 0 0 1 

This sets the parallel interface to write mode. Thus using the _outp function, data is written to the data 

port. 

_outp(DATA_PORT,0xA7); 

D7 D6 D5 D4 D3 D2 D1 D0 

1 0 1 0 0 1 1 1 



This above program is modified to include a function which handles the write to the parallel port and also 

the header is modified to include the function prototypes for the inpout32.dll required use on Windows 

NT/2000/XP: 

Example: Port_out.cpp 

// Program to initialise parallel port to write to an Application Card 



#include 

#include 













// Function Prototype 

int write_pport( int data); 

int main() 

{ 

write_pport(0xA7); 

// write 0xA7 to parallel port 

} 

return 0; 

// Function to write to the parallel port 

void write_pport( int data ) 

{ 

_outp(CNTRL_PORT,0x00); // reset bit 0 on Control Register 

_outp(CNTRL_PORT,0x01); // set bit 0 on Control Reg -> write data 

_outp(DATA_PORT,data); // write data to data port 

_outp(CNTRL_PORT,0x00); // reset bit 0 on Control Register 

} 



3.2.3 Input and Output Data 

Consider the following example, which reads in data from the parallel port and writes this data back out to 

the parallel port. 

Example 1: read_write_port.cpp 

// Program to initialise parallel port to write to and read from an 

// Application Card 



#include 

#include 













int main() 

{ 

int data; 


data = _inp(DATA_PORT); 



_outp(DATA_PORT,data); 

// set bit 5 on Control Reg -> read data 

// byte read from the Data Port 

// reset bit 5 on Control Register 

// set bit 0 on Control Reg -> write data 

// write data to data port 

} 

return 0; 

Exercise: 

Modify this example, to use functions for read and write to parallel port as used in the previous section/ 



3.3 Exercises 

Given the following circuit diagram showing a temperature sensor connected via ADC to the parallel port 

of the PC, write a C++ function to read the temperature and return the temperature value as an integer to 

the calling function. 

Analog 

votage 

0-2.55V 

Digital 8-bit 

0x00 to 0xFF 

Parallel Port 

Temperature 

Sensor 

ADC 

Data Reg 

0x378 

Control Reg 0x37A 

00100000 

Bit 5 

set for input 

Figure 10: Applications Board Inputs 

Given the following circuit diagram showing a Fan and heater connected via parallel port to the PC, write 

two C++ functions to 

• turn on the heater 

• turn on the fan 

LEDS 

7 6 5 4 3 2 1 0 

Data Reg Bit 6 or 7 -> Motor on 

Data Reg Bit 5 ->. Heater on 

Direction 

Relay 

Parallel Port 

Data Reg 

0x378 

MOTOR 

HEATER 

Control Reg 0x37A 

00000001 

Bit 0 

set for 

output 

Figure 11: Applications Board Outputs

Computer Systems 2: Hardware Interfacing 1. Introduction

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