1.2 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.

 

 

1.2.1 GPIB Bus (IEEE-488)

 

• 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.

 

 

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