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Computer Interfacing

Via the

Parallel Port

Carlos M. Oppus

ECCE Program, AdMU .

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Objectives

This lecture aims :

1) to give an overview of computer interfacing.

2) to give the basics of interfacing via the parallel port

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Introduction Computer Interfacing means

connecting different devices to the computer and being able to control or read the status of these devices.

There are many ways to achieve computer interfacing.

(1) Interfacing by making your own computer interface card.

(2) Interfacing via the existing ports in your computers.

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Introduction (cont …)

namely (a) modem (b) internet (c) serial port (d) game port (e) parallel port (f) USB etc.

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Parallel Port Computers are equipped with at least

one parallel printer port card. This parallel port can be used as a

general input-output device used for manipulating interfaced hardware.

The printer port is designated as LPT 1, LPT 2 & LPT 3

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Printer PortParallel (printer) port as a general-purpose

set of digital input and output port for interfacing devices.

 8-bit digital input/output (I/O) register if an

enhanced parallel port is being used otherwise it is just an output register

4-bit output register

4-bit input register (actually 5 inputs)

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Parallel (Printer) Port

Port Configuration I/O Port address

LPT 1 378H to 37AH

LPT 2 278H to 27AH

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8 output pins accessed via the DATA Port (pins 1 to 9) 5 input pins (one inverted) accessed via the STATUS Port (pins 10, 11, 12,

13, 15) 4 output pins (three inverted) accessed via the CONTROL Port (pins 1, 14,

16, 17)

The remaining 8 pins are ground (pins 18 to 25)

Parallel (Printer) Port

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Parallel (Printer) Portnote: high TTL level = 2.4V to 5V low TTL level = 0V to 0.8V (normally 0V)

An OUT instruction to port address 378H (278H) and 37A (27A) can write the data directly to the connector pins.

An IN instruction to port address 378H (278H) [if an enhanced parallel port is in use] and 379H (279H) can read the data directly from the connector pins.

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8-bit I/O if an enhanced parallel port is being used otherwise it is just an output port at address 378H (278H)

BIT TTL level when a 1 is written/read to/from

the port bit

pin position

description

0 high 2 D0 1 high 3 D1 2 high 4 D2 3 high 5 D3 4 high 6 D4 5 high 7 D5 6 high 8 D6 7 high 9 D7

example:  

PASCAL: port[$0378] := 255; // outputs 11111111 to pins 2 to 9

C: outportb(0x378,255); // outputs 11111111 to pins 2 to 9

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379H (279H)

4-bit input port at address 379H (279H) BIT TTL level when a

1 is read from the port bit

pin position

description

0 not used n/a n/a 1 not used n/a n/a 2 not used n/a n/a 3 not used n/a n/a 4 high 13 select 5 high 12 P.E. 6 high 10 ackn 7 low 11 busy

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PASCAL: data := port[$037A] ; reads port 37AH i.e. reads the TTL levels of

pins 13,12,10 & 11 respectively and places the corresponding values at the higher nibble of variable “data”.

C: data = inportb(0x37A); reads port 37AH i.e. reads the TTL levels of

pins 13,12,10 & 11 respectively and places the corresponding values at the higher nibble of variable “data”.

Example

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37AH (27AH)4-bit output port at address 37AH (27AH)

BIT TTL level when a 1 is written to the port bit

pin position

Description

0 low 1 strobe 1 low 14 autofeed 2 high 16 init prt 3 low 17 select inp 4 direction control bit of

data register (378H/278H) if enhanced parallel port (high for input, low for output)

not applicabl

e (n/a)

n/a

5 not used n/a n/a 6 not used n/a n/a 7 not used n/a n/a

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PASCAL:

Port[$037A] := 4; outputs 0100 to pins 17,16,14 & 1 respectively

i.e. pin 17 is high, pin 16 is high, pin 14 is high and pin 1 is high.

C:

outportb(x037A,4); outputs 0100 to pins 17,16,14 & 1

respectively i.e. pin 17 is high, pin 16 is high, pin 14 is high and pin 1 is high.

Example

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vbOut [port],[number] example:

'set port to 00001011 378H = 888vbOut 888, 11 [variable]=vbInp([port])

example:

‘input from 379H = 889PortNum%=vbInp(889)

For 32bit VB (WIN95IO.DLL), use: Declare Sub vbOut Lib "WIN95IO.DLL" (ByVal nPort As Integer, ByVal nData As Integer) Declare Sub vbOutw Lib "WIN95IO.DLL" (ByVal nPort As Integer, ByVal nData As Integer) Declare Function vbInp Lib "WIN95IO.DLL" (ByVal nPort As Integer) As Integer Declare Function vbInpw Lib "WIN95IO.DLL" (ByVal nPort As Integer) As Integer

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Sample Circuits

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Sample Circuits

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Demonstration

Appliance controller …

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Circuit Diagram

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Sample Program

uses crt;

var ch :char;

begin

clrscr;

repeat

port[$37A] := $0B; gotoxy(20,20); write('off');

ch := readkey;

port[$37A] := $04; gotoxy(20,20); write('on ');

ch := upcase(readkey);

until ch = 'Q';

port[$37A] := $0B;

end