4. Interfacing Computers for Data Transfer

4.1 Serial Data Transfer

• A single communication line links two computers, and allows communications one bit at a time.

• Typically strings are passed to and from a terminal, modem, mouse, etc.

• Each string is broken up and each byte is sent one at a time.

• Each byte is sent one bit at a time with the various framing options,

1 or 2 stop bits

Even, Odd, or No parity bit

7 or 8 data bits

1 start bit

 

• Advantages,

Very inexpensive

Easy to hook up

A popular communication port found on almost all computer hardware, regardless of age.

Can be robust in an industrial environment

Standards are very clearly defined

can use common phone lines

• Disadvantages,

Very slow

• Characteristics,

Speeds of 110, 150, 300, 600, 1200, 2400, 4800, 9600, 12000, 19200 baud (bits per second)

Can use as few as three wires for connection, or more as advanced handshaking lines are required.

Specialized chips, and modems are commercially available for using this interface

• Popular standards are RS (Recommended Standard)-XXX published by the Electronics Industries Association

RS-232: available on almost all computers

RS-422: a popular industrial standard which can be noise resistant then RS-232

RS-423

RS-449

4.2 Parallel Data Transfer

• Instead of sending the bits in a byte one at a time, parallel buses send the bits in parallel, so that the entire byte arrives at once.

• Basic parallel interfaces connect only two computers (see next section for other case)

• Advantages,

Can be very fast, and reliable

Easy to create computer hardware to support

Chips exist for easy implementation of this scheme

The parallel port may be used for alternate form of digital I/O

• Disadvantages,

Cabling can be more expensive

Standards are not as wide spread as serial communications

parallel ports are not universally available on computers and peripherals

• These interfaces have been popular for,

printer, and disk interfaces because of their higher speeds, and low costs

as a basic digital Input/Output source to drive indicator lights, keyboards, displays, etc.

• A Parallel bus that has been enhanced to support a number of computers connected by the same cable.

• A Brief History,

In the early 70’s there was a movement towards standard serial interfaces, but no clear development of a parallel interface standard. As a result Hewlett Packard (HP) set out to develop the GPIB (General Purpose Interface Bus).

The HP standard was accepted by both the IEEE and ANSI as standards in 1975.

By the early 80’s the standard was available in small personal computers (e.g. Commodore Pet Computers).

Today many products, and chips are available for development and use of the standard.

• Advantages,

Low costs

Widely available for test instruments

Maximum speeds between 500 Khz and 1 MHz

Can replace up to 16 individual serial interfaces with a single interface on the main computer

• Disadvantages,

Not necessarily real time,

Can be difficult for beginners to learn the bus architecture, but users are often isolated from this.

This is often used as a high performance interface on specialized equipment, but is not available on commercial applications anymore.

• Some details,

Each device on a GPIB bus has its own address number.

A talker-listener protocol is used to resolve bus usage

The devices on the bus can be instructed to identify themselves.