A ribbon cable is a cable with many conducting wires running parallel to each other on the same flat plane. As a result, the cable is wide and flat, rather than round like more "traditional" cables. Its name comes from the resemblance of the cable to a piece of ribbon (which is likewise wide and flat). Ribbon cables are common connectors for internal peripherals in computers, such as SCSI and ATA devices.

A two-conductor ribbon cable of 300 ohms characteristic impedance is commonly used as a transmission line for balanced connection of television antennas. This cable is also called twin-lead, see balun.

Cable sizes

For computer use, ribbon cables are usually specified by two numbers: the spacing or pitch of the conductors, and the number of conductors or ways. For example, the standard ATA cable used for PC hard disks and CD-ROM drives has 0.1 inch (2.54 mm) pitch and 40 ways, and the high-speed ATA cable has 0.05 inch (1.27 mm) pitch and 80 ways. Finer pitches, as small as 0.3 mm, are found in portable electronic equipment.

Cable connectors

Because of the large number of conductors or 'ways' in most ribbon cables, it is not practical to solder them individually to the pins of a connector. Instead, 'insulation displacement connectors' (IDC connectors) are used, in which the ribbon cable is forced onto a row of sharp forked contacts. (The phrase 'IDC connector' is widely used, even though it is redundant - an example of RAS syndrome.) Each contact pierces ('displaces') the insulation of one conductor and grips the copper core. Although it is sometimes possible to dismantle and re-use IDC connectors, they are not designed to allow this.

Popular types of connectors available with IDC termination include:

• BT224 connector - also defined by BS9525-F0023, DIN41651, MIL-C-83503 standards; these are the type used on ATA cables and are often simply called 'IDC connectors'. They mate with either a purpose-made plug or a two-row grid of header pins with 0.1 inch (2.54 mm) spacing.
• D-subminiature connector - used for serial ports and printer ports.
• DIN41612 connector - used for Eurocard buses.
• RJ-xx connectors - used on telephone and Ethernet cables.