−48 V Hot Swap Controller and Digital ( 52 Page)
Rev. A | Page 28 of 52
C bus is a common, simple serial bus used by many devices
to communicate. It defines the electrical specifications, the bus
timing, the physical layer, and some basic protocol rules.
SMBus is based on I
C and aims to provide a more robust and
fault-tolerant bus. Functions such as bus timeout and packet
error checking are added to help achieve this robustness, along
with more specific definitions of the bus messages used to read
and write data to devices on the bus.
PMBus is layered on top of SMBus and, in turn, on I
C. Using the
SMBus defined bus messages, PMBus defines a set of standard
commands that can be used to control a device that is part of a
command set is based upon the PMBus™ Power
System Management Protocol Specification, Part I and Part II,
Revision 1.2. This version of the standard is intended to provide
a common set of commands for communicating with dc-to-dc
type devices. However, many of the standard PMBus commands
can be mapped directly to the functions of a hot swap controller.
Part I and Part II of the PMBus standard describe the basic
commands and how they can be used in a typical PMBus setup.
The following sections describe how the PMBus standard and
specific commands are used.
is available in two models: the
. The PMBus address is seven bits in size. The
upper five bits (MSBs) of the address word are fixed and are
different for each model, as follows:
: Base address is 00100xx (0x10)
: Base address is 00110xx (0x18)
have a single ADR pin that
is used to select one of four possible addresses for a given
model. The ADR pin connection selects the lowest two bits
(LSBs) of the 7-bit address word (see
Table 6. PMBus Addresses and ADR Pin Connection
Value of Address LSBs
ADR Pin Connection
Connect to VEE
150 kΩ resistor to VEE
No connection (floating)
Connect to VCAP
SMBus PROTOCOL USAGE
C transactions on the
are performed using
SMBus defined bus protocols. The following SMBus protocols
are implemented by the
PACKET ERROR CHECKING
PMBus interface supports the use of the packet
error checking (PEC) byte that is defined in the SMBus standard.
The PEC byte is transmitted by the
during a read
transaction or sent by the bus host to the
write transaction. The
supports the use of PEC with
all the SMBus protocols that it implements.
The use of the PEC byte is optional. The bus host can decide
whether to use the PEC byte with the
on a message-
by-message basis. There is no need to enable or disable PEC in
The PEC byte is used by the bus host or the
errors during a bus transaction, depending on whether the trans-
action is a read or a write. If the host determines that the PEC
byte read during a read transaction is incorrect, it can decide to
repeat the read if necessary. If the
determines that the
PEC byte sent during a write transaction is incorrect, it ignores
the command (does not execute it) and sets a status flag.
Within a group command, the host can choose to send or not
send a PEC byte as part of the message to the
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