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. 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.
To reduce the risk of reversed connections—which could potentially damage hardware—either when making a cable or when using a cable with unpolarised 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 that 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 cable, 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). It is often known affectionately to its users as "hippie cable" due to its distinct appearance. However, this has remained a specialized and relatively expensive product.
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-disk-drive cables and older or custom Parallel ATA
cables, as well as many more specialized applications.
The "number of conductors" is usually restricted to a few values,
These include 4,6,8,9,10,14,15,16,18,20,24,25,26,34,37,40,50,60,64 and 80.
The rare 12 pin flatcable can be found at 3M
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).
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, although 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 to be done easily.
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.)
When electronics hobbyists are working on their computers or digital musical keyboards to "mod" (modify) or "hack" them, they sometimes have to solder ribbon cables. Soldering ribbon cables can present a challenge to a hobbyist who has not been trained as an electronics technician. In some cases, hobbyists strip off the wire with a fine razor, and then separate the wires before soldering them. Some hobbyists use fine sandpaper to wear away the plastic insulation from the wires. The sanding also primes the copper tracks. Then when the "tinned" soldering iron is touched onto the bare wire, the solder is guided into the track.
From a digital point of view, ribbon cable is an ideal way to connect two devices. However, from an analog point of view, these cables are problematical. Around 1980, the FCC
discovered that ribbon cables were highly efficient antennas
, broadcasting essentially random signals across a wide band of the electromagnetic spectrum
. These unintended signals could interfere with domestic TV reception, putting "snow" on the screen. The FCC issued edicts and injunctions to the personal-computer industry, restricting the use of ribbon cables to connect devices together. "Naked" ribbon cable could be used inside the case
of a computer or peripheral device, but any ribbon cable connecting two boxes together had to be grounded
. This rule led to solutions such as ribbon cables covered by a copper-braid shield, which made it impossible to see or separate the individual connectors. On the Apple II
, these cables passed through the holes on the back of the computer that were grounded to the power supply. Eventually, ribbon connectors were replaced, for inter-connect purposes, by a wide profusion of custom-designed round cables with molded connectors.