spp.zip_in_spp

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Interfacing the Standard Parallel Port

Disclaimer : While every effort has been made to make sure the information in this document is correct, the author can not be liable

for any damages whatsoever for loss relating to this document. Use this information at your own risk.

Table of Contents

Introduction to Parallel Ports Page 1

Parallel Port Modes in BIOS Page 14

Parallel Port Modes and the ECP’s Extended Control Register Page 15

Introduction to Parallel Ports

The Parallel Port is the most commonly used port for interfacing home made projects. This

port will allow the input of up to 9 bits or the output of 12 bits at any one given time, thus requiring

minimal external circuitry to implement many simpler tasks. The port is composed of 4 control lines,

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Interfacing the Standard Parallel Port Page 2

also backwards compatible with the Standard Parallel Port (SPP). Compatibility, Nibble & Byte

modes use just the standard hardware available on the original Parallel Port cards while EPP & ECP

compatible with the Standard Parallel Port.

data which is written will be lost.

3. Take the Strobe (Pin 1) low. This tells the printer that there is the correct data on the data

4. Put the strobe high again after waiting approximately 5 microseconds after putting the strobe

This limits the speed at which the port can run at. The EPP & ECP ports get around this by

1284 Type B which is the 36 pin Centronics Connector found on most printers.

IEEE 1284 Type C however, is a 36 conductor connector like the Centronics, but smaller. This

connector is claimed to have a better clip latch, better electrical properties and is easier to assemble. It

also contains two more pins for signals which can be used to see whether the other device connected,

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has power. 1284 Type C connectors are recommended for new designs, so we can look forward on

seeing these new connectors in the near future.

1 1 nStrobe In/Out Control Yes

2 2 Data 0 Out Data

3 3 Data 1 Out Data

10 10 nAck In Status

11 11 Busy In Status Yes

12 12 Paper-Out

In Status

13 13 Select In Status

14 14 nAuto-Linefeed In/Out Control Yes

15 32 nError / nFault In Status

16 31 nInitialize In/Out Control

17 36 nSelect-Printer

ASIC, can sink and source around 12mA. However these are just some of the figures taken from Data

sheets, Sink/Source 6mA, Source 12mA/Sink 20mA, Sink 16mA/Source 4mA, Sink/Source 12mA.

As you can see they vary quite a bit. The best bet is to use a buffer, so the least current is drawn from

the Parallel Port.

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Centronics?

Centronics is an early standard for transferring data from a host to the printer. The majority of

printers use this handshake. This handshake is normally implemented using a Standard Parallel Port

under software control. Below is a simplified diagram of the ‘Centronics’ Protocol.

Port Addresses

The Parallel Port has three commonly used base addresses. These are listed in table 2, below.

The 3BCh base address was originally introduced used for Parallel Ports on early Video Cards. This

configuration can be changed using BIOS.

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If a Parallel card is found there, it is assigned the next free device label. This would be LPT1 if a card

and not at the expected address 278h.

devices, it stores the address at specific locations in memory, so we can find them.

PS/2 and newer Bioses, and thus this field may be invalid.