PMBus

The Power Management Bus (PMBus) is a variant of the System Management Bus (SMBus) which is targeted at digital management of power supplies. Like SMBus, it is a relatively slow speed two wire communications protocol based on I²C. Unlike either of those standards, it defines a substantial number of domain-specific commands rather than just saying how to communicate using commands defined by the reader.

Overview

See the PMBus 1.1 specification for full details. The first part gives an overview with particular reference to SMBus, while the second part goes into detail about all the commands defined for PMBus devices. There are both standardized commands and manufacturer specific commands. Conformance requirements for PMBus are minimal, and are described in Part I of the specification.

Comparison to SMBus

At the lowest level, PMBus follows SMBus 1.1 with a few differences. This information is presented in more detail in Part I of the PMBus specification:
* 400 kHz bus speeds are allowed (vs. the 100 kHz limit of SMBus)
* In PMBus, blocks may include up to 255 bytes (vs. the 32 byte limit of SMbus).
* As in SMBus 2.0, only seven bit addressing is used.
* Some commands use the SMBus 2.0 block process calls.
* Either the SMBALERT# mechanism or the SMBus 2.0 host notify protocol may be used to notify the host about faults.
* PMBus devices are required to support a Group Protocol, where devices defer acting on commands until they receive a terminating STOP. Since commands can be issued to many different devices before that STOP, this lets the SMBus master synchronize their actions.
* An "extended command" protocol is defined, using a second command byte to add 256 more codes each for both standard and manufacturer-specific commands.

PMBus commands

You may think of the PMBus command space as exposing a variety of readable, and often writable, device attributes such as measured voltage and current levels, temperatures, fan speeds, and more. Different devices will expose different attributes. Some devices may expose such attributes in multiple "pages", as for example one page managing each power supply rail (maybe 3.3V, 5V, 12V, -12V, and a programmable supply supporting 1.0-1.8V). The device may set warning and fault limits, where crossing a limit will alert the host and possibly trigger fault recovery. Different devices will offer different capabilities.

The ability to query a PMBus 1.1 device about its capabilities may be particularly useful when building tools, especially in conjunction with the ability to store user data in the devices (e.g. in EEPROM). Without such a query capability, only error-prone external configuration data is available.

Part II of the PMBus specification covers every standard PMBus command. It also describes the models for managing output power and current, managing faults, converting values to and from the formats understood by a given device, and accessing manufacturer-provided information such as inventory data (model and serial number, etc) and device ratings.

Products

As of summer 2007, PMBus is relatively new so not many products yet advertise support for it. Given the richness of the specification, firmware based implementations running in microcontrollers are probably easiest to provide, although several of the current products do not involve microcontrollers. One firmware-based example is the Texas Instruments [http://focus.ti.com/docs/prod/folders/print/ucd9111.html UCD9111]

As PMBus systems are deployed, tools to manage those systems should become significant. Some of them may just be used during manufacturing, to set up system-specific parameters used with reconfigurable power subsystems. Others will be useful for runtime optimization, for example with server farms.

See also

* I2C
* SMBus

External links

* [http://www.pmbus.org PMBus.org] hosts the specifications summarized above
* [http://www.electronicproducts.com/ShowPage.asp?FileName=artesyn.aug2005.HTML PMBus–panacea or hype?] is an article that's informative about the context of PMBus, written by the specifications' editor