PMBus - Power Management Bus

- notes & overview of the basics of PMBus, Power Management Bus technology, detailing what it is and how it may be used.

The Power Management Bus, PMBus is an open standard protocol used for power management applications.

The Power Management Bus, PMBus is a fully open standard and has a language with commands that enables communication with power converters, regulators or other elements within a power management system.

With systems becoming more complicated, and with the growing need to become more power efficient, the Power Management Bus provides a method of enabling power systems to react to the state of a system more effectively so that power can be provided as and when it is needed.

PMBus-IF

The PMBus-IF or PMBus Implementers Forum was set up to provide support and facilitate the adoption of PMBus.

In this role it publishes the PMBus standard and gathers together interested parties within the industry so that they can work together to provide a common way of working.

It has been proved that open standards promote a general technology area, and this is of benefit to all.

The PMBus standard was set up because it was realised that the lack of a suitable standard was inhibiting he development of an all-digital power management system. By developing a common standard all manufacturers and users would be able to benefit from the additional facilities and flexibility provided.

PMBus basics

The Power Management Bus, PMBus is an open standard digital power management protocol. It is based upon another system called the System Management Bus, SMBus. This was originally developed by Intel for system management applications in servers and PCs. This was an improvement of the I2C - Inter IC communication standard that was used for communication between ICs in a system.

The PMBus, is targetted at the digital management of power supplies and provides many simplifications of the SMBus not required within power management applications. Its aim is to enable power supplies and other power management systems to operate more effectively.

The PMBus, Power Management Bus facilitates communication between a power converter and another device or devices. It fully defines both the transport and physical interfaces as well as the command language needed to accomplish this.


Parameter Details
Bus speed Clock speeds of up to 400 kHz are allowed
Data block size Up to 255 bytes
Addressing 7 bit addressing is used
Command language A command language is defined within the PMBus standard. It is not just an interface specification.
Basic protocol PMBus devices are support a Group Protocol. Devices defer acting on commands until they receive a terminating STOP.
Extended protocol An "extended command" protocol is defined. This uses a second command byte to add 256 more codes each for both standard and manufacturer-specific commands

PMBus command language

One of the key elements of the PMBus is that it not only defines the interface, but it also defines the command language. As a result, it is possible for communications to be effected regardless of the manufacturer.

The PMBus communications are based on a simple command set. Each packet consists of the following elements in order in the data packet:

  • Address byte
  • Command byte
  • Zero, one or more data bytes
  • Optional packet error code, PEC

By Ian Poole

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