SYSTEM MANAGEMENT BUS

9/20/2003

System Management Bus (SMB or SMBus). The SMB is a two (three) wire -- data, clock (,ground) -- interface to simple system monitoring devices. The SMB was defined in 1995 and was introduced in PCs in the Intel 430TX chipset. The SMB provides simple, low speed, serial data communications based on Phillips I2C protocol. As of 2003, the current SMB specification is either 1.1 or 2.0 depending on which web pages you believe.

SMB is somewhat interoperable with the Phillips I2C Bus. It transmits data at 100KHz -- one of the permitted I2C speeds. Specified signal voltage levels differ, but not so much so that devices designed for one bus won't work on the other. The bus is bidirectional and permits devices to operate as either masters or slaves. Each device has a unique address. Messages have an eight bit CRC.

Typical usage of the SMBus is to monitor battery state and charging in portable PCs; to recognize memory modules via the SPD proms; to monitor the state of case lid switches, and to monitor the temperature of CPUs.

The SMB is apparently supported by most chipsets and BIOSes. Windows drivers for the System Management Bus are generally included with the motherboard drivers. The SMB is frequently reported by Windows as an "Unknown Device". The Windows 9 family does not support the SMBus at all. Windows 2000, Windows XP, and Linux support the SMBus. It appears that very often the functions accessed by the SMB do not need Operating System support in any case and the lack of drivers is not a problem.

http://www.smbus.org/

Modified 040925

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