Parallel Interface

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Before you start

Objectives: learn what is parallel interface and how does it work.

Prerequisites: no prerequisites.

Key terms: cable, port, data, eight, 8, wire, length, transfer, bi-directional, connector, ibm, ieee, printer, signal, standard


How Does it Work

With a serial interface we had a single signal traveling one bit at a time on the wire. With a parallel interface instead of having one wire carrying data, we have several wires. Because of that we can transfer data not one bit at a time but several bits at a time. On parallel interfaces we have 8 data transmission wires, so the parallel interface can transfer a single byte at once.

Parallel Interface

Image 316.1 – 8 bits sent at a time

The advantage of a parallel interface over a serial interface is speed. In theory it should be eight times faster then a serial interface because we can use eight data wires at a time. One of the problems with the parallel interface was cable length. When we’re dealing with a parallel interface we’re transferring eight signals across eight wires all at one time. Because of this, it is possible that some bits on some wires get sooner to the destination (or later), instead of all bits arriving at the same time. If that happens, the receiving device doesn’t know where the byte begins and where they byte ends. This problem is called signal jitter and it’s very common with parallel interfaces. To reduce signal jitter we decrease the length of the cable.

Parallel devices use LPT ports for resources. Parallel ports on the back of a computer are always female connectors.

Parallel Port

Image 316.2 – Parallel Port

Printers were common parallel devices. There are two common types of parallel cables. The first is an IBM Printer cable which is a uni-directional cable with a 25-pin male DB25 connector at one end and a 36-conductor male Centronics connector at the other end.

IBM PPC

Image 316.3 – IBM Parallel Printer Cable

The second cable is IEEE 1284, which is a newer, bi-directional cable. Connectors used are similar to the IBM printer cable, but with different connector genders possible.

IEEE 1284 Cable

Image 316.4 – IEEE 1284 Cable

The maximum specified length of IEEE 1284 is 10 meters (30 feet), and data can be transferred at up to 2 MB/s. This cable is more commonly used. It also specifies a daisy chaining standard that allows up to eight devices to be connected to a single parallel port.

Standards for Parallel Communication

The original parallel port standard is Standard Parallel Port (SPP). It could transfer data in a single direction (only from the computer to the destination device). The speed was from 50 to 150 kilobytes per second.
Cables used with SPP should not exceed 10 feet (3 meters) in length.

The second protocol is called Enhanced Parallel Port (EPP), and it could transfer 500 kilobytes per second to 2 megabytes per second. It supports bi-directional communications, and was commonly used by non-printing devices.

The third protocol is Extended Capabilities Port (ECP) which requires an ECP chip on the system board. It can achieve high data rates for input rather than output. It supports bi-directional communications as well as DMA. Usually our motherboard will support all three modes. If that is the case, we can edit parallel port mode in our BIOS settings.

Remember

With a parallel interface we use 8 wires to transfer data. Parallel interface is faster than serial interface. Common problem with parallel interfaces is signal jitter. Parallel ports on the back of a computer are always female connectors.