Field Replaceable Unit

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A Field Replaceable Unit or FRU is a circuit board, part, or assembly that can be quickly and easily removed from a personal computer or other piece of electronic equipment and replaced by the user or by a technician without having to send the entire product or system to a repair facility.

It should be noted that FRU's are not strictly confined to computers but are also apparent in many high-end, lower volume consumer and commercial product. The main content of this article, however, will be concerned with that of FRU's in computers.

FRUs allow a technician without in-depth knowledge to determine faulty parts by the process of elimination.

Nearly every component of an x86 computer is an FRU. Typical FRUs include:

Motherboards
CPUs
RAM modules
System drives, such as floppy, hard drives, and optical drives
Bus devices, such as video cards and sound cards
Power supply units
Peripherals, such as keyboards, mice, printers, and the cables connecting them

Replacing an FRU while a machine is running is hot swapping.

Many vacuum tube computers had FRUs:

Pluggable units containing one or more vacuum tubes and various passive components

Most transistorized and integrated circuit computers had FRUs:

Circuit boards containing discrete transistors and various passive components. Some examples being:
- IBM Standard Modular System (SMS) cards
- DEC System Building Blocks cards
- DEC Flip-Chip® cards
Circuit boards containing monolithic ICs and/or hybrid ICs. Some examples being:
- IBM Solid Logic Technology (SLT) cards
- DEC Flip-Chip® cards

Vacuum tubes themselves are usually FRUs.

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

Field Replaceable Unit Identification Devices (FRU ID) holds the records of the devices that does not originally come along with the baseboard or motherboard. Most of the board manufacturers use EEPROM (but not mandatory) to store those informations. It has a very great importance in IPMI. The BMC contacts the FRU devices whenever there is a problem in the hardware and it wants to know which device to be replaced. The BMC communicates with the FRU basically using [I²C] bus protocol [Philips]. It can also use other protocols, but it is implementation specific.