Master-slave (computers)

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This page is about the computer protocol; for information on the general terms see Master and Slave. For the philosophical concept, see Master-Slave.

Master/slave is a model for a communication protocol where one device or process has unidirectional control over one or more other devices.

Once a master/slave relationship between devices or processes is established, the direction of control is always from the master to the slaves.

In some systems a master is elected from a group of eligible devices, with the other devices acting in the role of slaves.[1][2]

1 Examples
2 Controversy
3 See also
4 References

[edit] Examples

Peripherals connected to a bus in a computer system.
A series of diesel locomotives is often used in tandem to pull heavy loads, or to ascend and descend steep mountains. The first engine in the front is the master, and the other engines (often completely without a cab with controls for the engineer) are the slaves.
Duplication is often done with several cassette tape or compact disc recorders linked together. Operating the controls on the master triggers the same commands on the slaves, so that recording is done in parallel.
In parallel ATA hard drive arrangements, the terms master and slave are used but neither drive has control over the other. The terms also do not indicate precedence of one drive over the other in most situations. "Master" is merely another term for device 0 and "slave" indicates device 1.

[edit] Controversy

Sometimes the terms master and slave are deemed offensive.

On November 2003, the County of Los Angeles sent an e-mail to its suppliers asking them not to use these terms:

Subject: IDENTIFICATION OF EQUIPMENT SOLD TO LA COUNTY
Date: Tue, 18 Nov 2003 14:21:16 -0800
From: "Los Angeles County"

The County of Los Angeles actively promotes and is committed to ensure a work environment that is free from any discriminatory influence be it actual or perceived. As such, it is the County's expectation that our manufacturers, suppliers and contractors make a concentrated effort to ensure that any equipment, supplies or services that are provided to County departments do not possess or portray an image that may be construed as offensive or defamatory in nature.

One such recent example included the manufacturer's labeling of equipment where the words "Master/Slave" appeared to identify the primary and secondary sources. Based on the cultural diversity and sensitivity of Los Angeles County, this is not an acceptable identification label.

We would request that each manufacturer, supplier and contractor review, identify and remove/change any identification or labeling of equipment or components thereof that could be interpreted as discriminatory or offensive in nature before such equipment is sold or otherwise provided to any County department.

Thank you in advance for your cooperation and assistance.

Joe Sandoval, Division Manager
Purchasing and Contract Services
Internal Services Department
County of Los Angeles

Many in the Information Technology field rebuff this claim of discrimination and offence as ridiculous, noting that the master/slave terminology accurately reflects what is going on inside the device and that this was not intended in any way to be a reference to slavery as it existed in the United States.

Recently, there were rumors of a major push to change the way hardware manufacturers refer to these devices. It has not had much effect on most of the products being produced. It may not matter in a few years, with SATA replacing older IDE (PATA) drives. This standard allows only one drive per connection, and does not require the use of master/slave terms.