This article is primarily about FRUs in computers.

A field replaceable unit ('FRU) is a circuit board, part or assembly that can be quickly and easily removed from a personal computer

Personal computer

A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...

 or other piece of electronic equipment, and replaced by the user or a technician without having to send the entire product or system to a repair facility. FRUs allow a technician lacking in-depth product knowledge to fault isolate

Fault detection and isolation

Fault detection and isolation is a subfield of control engineering which concerns itself with monitoring a system, identifying when a fault has occurred, and pinpointing the type of fault and its location...

 and replace faulty components.

Other equipment

FRUs are not strictly confined to computers but are also part of many high-end, lower volume consumer and commercial products. For example, in military aviation, electronic components of line-replaceable unit

Line-replaceable unit

A line-replaceable unit is a modular component of an airplane, ship or spacecraft that is designed to be replaced quickly at an operating location. An LRU is usually a sealed unit such as a radio or other auxiliary equipment...

s, typically known as shop-replaceable units (SRUs), are repaired at field-service backshops, usually by a "remove and replace" repair procedure, with specialized repair performed at centralized depot or by the OEM

Original Equipment Manufacturer

An original equipment manufacturer, or OEM, manufactures products or components that are purchased by a company and retailed under that purchasing company's brand name. OEM refers to the company that originally manufactured the product. When referring to automotive parts, OEM designates a...

.

History

Many vacuum tube

Vacuum tube

In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

 computers had FRUs:

• Pluggable units containing one or more vacuum tube
  Vacuum tube
  In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
  
  s and various passive components

Most transistor

Transistor

A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...

ized and integrated circuit

Integrated circuit

An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

-based computers had FRUs:

• Circuit boards containing discrete transistors and various passive components. Some examples:
  • IBM SMS cards
  • DEC System Building Blocks cards
  • DEC Flip-Chip
    Flip Chip (trademark)
    Flip-Chip modules were used in the DEC PDP-7 , PDP-8, PDP-9 and PDP-10, beginning on August 24, 1964. The trademark "Flip-Chip"' was filed on August 27, 1964...
    
     cards
• Circuit boards containing monolithic ICs and/or hybrid ICs
  Hybrid circuit
  A hybrid integrated circuit, HIC, hybrid microcircuit, or simply hybrid is a miniaturized electronic circuit constructed of individual devices, such as semiconductor devices and passive components , bonded to a substrate or printed circuit board...
  
   such as IBM SLT cards
  Solid Logic Technology
  Solid Logic Technology was IBM's method for packaging electronic circuitry introduced in 1964 with the IBM System/360 series and related machines. IBM chose to design custom hybrid circuits using discrete, flip chip-mounted, glass-encapsulated transistors and diodes, with silk screened resistors...
  
  .

Vacuum tubes themselves are usually FRUs.

For a short period starting in the late 1960s some television

Television

Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...

manufacturers started making solid-state televisions with FRUs instead of a single board attached to the chassis. However all modern televisions put all the electronics on one large board to reduce manufacturing costs.

Trends

As the sophistication and complexity of multi-replaceable unit electronics in both commercial and consumer industries have increased, many design and manufacturing organizations have expanded the use of the FRU storage device. Storage is no longer limited to simply identification of the FRU itself, but now also comprises back-up copies of critical system information such as system serial numbers, MAC address and even security information. Some systems will fail to function at all without each FRU in the system being ratified at start-up. Today one cannot assume that the FRU storage device is only used to maintain the FRU ID of the part.