Serial and Parallel data communications
These are two different and competing approaches to transmitting data using a physical cable. The idea can be extended to wireless communication too. The underlying idea is simple:
- The fundamental unit of data is the byte.
- The byte is eight bits.
- Each bit is an on/off signal which takes a certain amount of time to transmit
- Do we send a bit at a time, or a byte?
Serial - one bit at a time
Serial cables contain only two wires - a signal and a ground, which transmit the information. Bytes coming from the computer's bus need to be disassembled into their individual bits (by a device called a UART - Universal Asynchronous Receiver and Transmitter). Those bits are then sent by turning them into electrical pulses on the signal wire.
At the other end the received bits are rebuilt into the original data byte. |
Parallel - one byte at a time
Parallel cables contain eight data wires, together with some additional wires for control. The byte of data is kept intact and sent down the eight wires at the same time.
The illustration above is probably as close as you'll get to a parallel cable, we couldn't find one for the lesson in the whole school. Imagine it's thick, heavy and about a meter long. |
Why did serial win?
Parallel cables sound much better in principle than serial cables. After all there are eight times as many data wires and less dismantling and rebuilding taking place. In practice it's almost impossible to find a parallel cable in use now - although you may occasionally see them on older printers. Most PC motherboards do not have a parallel port. Macintosh computers have not had parallel ports since the 1990s. Why?
- Expense: Parallel cables are much more expensive than serial cables. They have many times more copper per meter and they are much harder to make.
- Crosstalk: The wires in a parallel cable interfere with each other electrically. This severely limits the practical length of a parallel cable to about one meter.
- Performance: The crosstalk problem becomes much worse at high speeds. As data transmission rates have increased, parallel cable technology has not been able to keep up and the crosstalk problems have not been solved. As a result parallel cables do not have any performance advantages over serial cables, they're just more expensive.
Parity - simple error checking
When data is sent across a cable, errors can creep in. Electrical noise is the main reason, this can be interpreted as data and corrupt the signal.
Parit is a simple way of adding one additional byte to a message which acts as a check on the others. See how it works in this demonstration.
Parit is a simple way of adding one additional byte to a message which acts as a check on the others. See how it works in this demonstration.
|
|
The parity trick
|
