Left: 20-way grey ribbon cable with wire no. 1 marked red, insulation partly stripped. Right: 16-way rainbow ribbon with IDC connector.

A ribbon cable (also known as multi-wire planar 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. Its name comes from the resemblance of the cable to a piece of ribbon (which is likewise wide and flat).

Ribbon cables are commonly seen for internal peripherals in computers, such as hard drives, CD drives and floppy drives. On some older computer systems (such as the BBC Micro) they were commonly used for external connections as well. Unfortunately the ribbon like shape makes them awkward to handle especially when there are a lot of them and so round cables have almost entirely replaced ribbon cables for external connections.

Color-coding

To reduce the risk of reversed connections (which could potentially damage hardware) either when making a cable or when using a cable with non polarised connectors one edge of the cable is usually marked with a red stripe. By convention the edge with the stripe is connected to pin 1 on the connector. This method of identification is fine for cables which just consist of two or more IDC connectors with every connector connecting to every wire but is somewhat less helpful when individual wires or small groups of wires must be terminated separately.

To make it easier to identify individual conductors in a cable; ribbon cable manufacturers introduced rainbow ribbon cables which uses a repeating pattern of colors borrowed from the standard resistor color code (Brown is pin 1 or pin 11 or pin 21 etc. Red is pin 2 or pin 12 or pin 22 etc). However this has remained a specialized and relatively expensive product.

Cable sizes

Ribbon cables are usually specified by two numbers: the spacing or pitch of the conductors, and the number of conductors or ways. Conventionally a spacing of 0.05 inch (1.27 mm) was the norm allowing for a two row connector with a pin spacing of 0.1 inch (2.54 mm). This size is still used today in floppy cables and older or custom Parallel ATA cables as well as many more specialist applications.

The high-speed ATA cable used for ULTRA-ATA 66 and above has 0.025 inch (0.64 mm) pitch and 80 ways. The 40 pin connection is still used but with special connectors that ground every other wire in the 80 way cable. Finer pitches, as small as 0.3 mm, are found in portable electronic equipment such as laptops, however portable electronic equipment usually uses FFC(Flexible Flat Cables).

Cable connectors

The main point of ribbon cables is to allow mass termination to specially designed insulation displacement connectors (IDC connectors) 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.) Most commonly this is done at both ends of the cable though sometimes (for example when making a lead that needs to change wiring between the two connectors) only one end will be IDC terminated with the other end being terminated in a regular crimp or solder bucket connection. 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 suitable for ribbon cable 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 (however IDC D connectors are far less common than crimp and solder bucket types).
• DIN41612 connector - used for Eurocard buses.
• PCB transition headers - has two rows of pins with the same spacings as BT244 connectors. Intended to be soldered directly into a PCB.
• DIL headers - Has pins with the same spacings as standard DIL ICs. Generally used where for some reason it is desired to replace an IC with a connection to an external device (e.g. in-circuit emulators). Can also be used like a PCB transition header especially on stripboard. (Fitting a standard spacing header to stripboard is tricky because you have to cut the tracks between two holes rather than on a hole.)

See also

• FFC - Flat Flex Cable

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