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. Its
name comes from its resemblance to a piece of ribbon.
Ribbon cables are usually seen for internal peripherals in computers, such
as hard drives, CD drives and floppy drives. On some older computer systems
they were used for external connections as well. The ribbon-like shape
interferes with computer cooling by disrupting airflow within the case and
also makes the cables awkward to handle, especially when there are a lot of them;
as a result, round cables have almost entirely replaced ribbon cables for
external connections and are increasingly being used internally as
well. History
The ribbon cable was invented in 1956 by Cicoil Corporation, a company based in
Chatsworth, California. The company's engineers figured out how to use a new
material, silicone rubber, to 'mold' a flat cable containing multiple
conductors of the same size. Since the cable looked like a flat ribbon or tape,
it was named a ribbon cable. The ribbon cable allowed companies like IBM and
Sperry/Univac to replace bulky, stiff round cables with sleek, flexible ribbon
cables. The early ribbon cables were used in the
mainframe computer industry, on card readers, card punching machines, and
tape machines. Subsequently ribbon cables were manufactured by a number of
different companies, including 3M. Methods and materials were developed to
simplify and reduce the cost of ribbon cables, by standardizing the design and
spacing of the wires, and the thickness of the insulation, so that they could be
easily terminated through the use of insulation displacement connectors. Due
to the simplicity of ribbon cables, their low profile, and low cost due to
standardization, ribbon cables are used today in most computers, printers, and
many electronic devices. During the 1960s and 1970s the company
provided flat cables for NASA and the US Government. In the 1990s Cicoil
developed a unique extrusion process to make ribbon cables and flat flexible
cables out of wires, hollow tubing, coaxial cable, and fiber optics. These
cables are used in applications including missiles, satellites,
semiconductor manufacturing equipment, and medical equipment.
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 unpolarized 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. 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.
Sizes Ribbon cables are usually specified by
two numbers: the spacing or pitch of the conductors, and the number of conductors
or ways. A spacing of 0.05 inch is the most usual, allowing for a two-row
connector with a pin spacing of 0.1 inch. These types are used for many
types of equipment, in particular for interconnections within an enclosure.
For personal computers, this size is used today in floppy-disk-drive cables
and older or custom Parallel ATA cables. Based on availability of standard
connectors, 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 wire is usually stranded copper wire, usually either
0.32, 0.20, or 0.13 mm2. Finer and coarser pitch cables are also
available. For instance, the high-speed ATA interface cable used for computer
hard disk interfaces ULTRA-ATA has 0.025-inch pitch. Finer pitches, as
small as 0.3 mm, are found in portable electronic equipment, such as laptops;
however, portable electronic equipment usually uses flexible flat cables.
Connectors The main point of ribbon cables is to
allow mass termination to specially designed IDC connectors in which the
ribbon cable is forced onto a row of sharp forked contacts. Most commonly
termination is done at both ends of the cable, although sometimes 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 spacing. D-subminiature connector – used for
serial ports and printer ports. Micro ribbon connector - used for 36-pin
printer ports and 50-pin SCSI ports. DIN 41612 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. Can also be used like a
PCB transition header, especially on stripboard.
When electronics hobbyists are working on their computers or digital musical
keyboards to "mod" 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. = Interference=
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 problematic.
Around 1980, the U.S. Federal Communications Commission 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.
= Impedance= One of the most popular sizes of ribbon
cable employs 26AWG wire. Using the common 0.050" spacing and common PVC
insulation the resultant impedance for any two adjacent wires within the cable
is; Z = 110 - 130. The precise number will vary a few percent due to
materials. Knowledge of the impedance is one step toward understanding and
control of interference that may be caused by ribbon cables.
= Proper Usage= According to NASA standards, the minimum
bend radius for short term uses should be no less than 6 diameters, and no less
than 10 diameters for long term use. See also
Flexible flat cable References
Product Design for Manufacture and Assembly, Third Edition page 143-144
"4.4 Types of wires and cables" Digital Systems Engineering, William J.
Dally, John W. Poulton, page 52 "2.7.2.2 Ribbon cable"