Intel System Management Bus

Abbreviated to SMBus or SMB, the System Management Bus is a single-ended simple two-wire bus for the purpose of lightweight communication. It is generally found in computer motherboards for communication with the power source for ON/OFF instructions.

It is derived from I²C for communication with low-bandwidth devices on a motherboard, especially power related chips such as a laptop’s rechargeable battery subsystem. Other devices might include fan or voltage sensors, temperature, lid switches and clock chips. PCI add-in cards may connect to an SMBus segment.

A device can indicate its model/part number, provide manufacturer information, report different types of errors, save its state for a suspend event, accept control parameters and return status. The SMBus is generally not user configurable or accessible, and although SMBus devices usually can’t identify their functionality, a new PMBus coalition has extended SMBus to include conventions allowing that.

Intel and Duracell defined the SMBus in 1994. It carries clock, data, and instructions and is based on Philips’ I²C serial bus protocol, with the clock frequency ranging from 10 kHz to 100 kHz. PMBus extends this to 400 kHz. Its voltage levels and timings are more strictly defined than those of I²C, but it is said through user experiences that the devices belonging to the two systems are often successfully mixed on the same bus. SMBus is used as an interconnect in several platform management standards which ASF, DASH, and IPMI.