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.