Field Replaceable Unit

Field Replaceable Unit

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 a technician without having to send the entire product or system to a repair facility.

It should be noted that FRUs are not strictly confined to computers but are also part of many high-end, lower volume consumer and commercial products. This article is primarily about FRUs in computers.

FRUs allow a technician lacking in-depth product knowledge to determine faulty parts by the process of elimination.

Nearly every component of an x86 computer is an FRU. Typical FRUs include:
*RAM modules
*System drives, such as floppy, hard drives, and optical drives
*Bus devices, such as video cards and sound cards
*Power supply units
*Cooling fans
*Peripherals, such as keyboards, mice, printers, and the cables connecting them

Replacing an FRU while the machine is running is known as hot swapping. Support for hot swapping is mostly limited to certain high-availability (HA) devices (such as disk drives in a RAID array), or power supplies and/or cooling fans in systems having redundant ones.

The Field Replaceable Unit IDentifier (FRU ID) holds the records of the devices that do not originally come with the baseboard or motherboard. Most (though not all) board manufacturers use EEPROM to store FRU IDs, which play a significant role in IPMI. The BMC contacts the FRU devices whenever there is a problem in the hardware and it wants to know which device needs replacement. Most often, the BMC communicates with the FRU using I²C bus protocol Philips, but can also use other implementation-specific protocols.

Recent Trends

As the sophistication and complexity of multi-replacable 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.


Many vacuum tube computers had FRUs:
*Pluggable units containing one or more vacuum tubes and various passive components

Most transistorized and integrated circuit-based computers had FRUs:
*Circuit boards containing discrete transistors and various passive components. Some examples:
**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:
**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.

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