Intelligent Chassis Management Bus

ICMB Description

Intelligent Chassis Management Bus [ICMB] is used as a control interface between chassis.
The ICMB is a multi-drop, multi-master, 2-wire, half-duplex, differential drive bus, using RS-485 transceivers.
RS485 allows the equipment chassis to communicate with one another over a cable interface.
Signaling is based on RS232 [standard UART character framing].
The bus is a broadcast medium; all receivers on it see the same bus state and transitions.
The bus is biased to the high (1) state when not explicitly driven by a bridge.

The ICMB has been targeted to support networks/buses with up to 42 chassis [64 chassis maximum]
(2-4 servers, the remainder being peripheral chassis) and up to 300 feet of cable.
Messages could include communicating information such as on board voltages, temperatures, fan rotation speed, processor and bus failures [including remote control].

EIA-485 Interface Circuit
EIA/TIA-485 Interface Circuit {RS485}

The ICMB uses intelligent controllers (microcontrollers) that function independently of the system's main processors. In addition, the controller and ICMB interface in each system is specified to stay powered while the main system is powered down. ICMB messaging and command sets are an extension of the IPMI specifications. Bridges are used to connect the incoming ICMB interface to the IPMB bus [internal to the chassis].

ICMB uses a 16-bit address, while the IPMB uses 7-bit addresses.

ICMB transceivers must all share a common ground, connected in a passive star, daisy-chain, or T-drop topologies. TIA/EIA RS-485 parallel termination is not used. Instead, a series termination is used. This is done to avoid the need for having termination resistors placed at the ends of the cable. The ICMB uses series termination to reduce the effect of signal reflections on the cable. Resistor values should be selected such that the output impedance of the driver in series with the termination resistors matches the cable impedance.

There are two connector types specified for the ICMB. These are referred to as the Type A and Type B connectors.
The type 'A' connector is a 6-pin modular connector.

6-pin modular Pin Out
Signal NamePin Number
No Connect2
Tx/Rx +3
Tx/Rx -4
Connector ID +5
Connector ID -6

Pins 5 and 6 are optional

The type 'B' connector is a keyed 8-pin RJ45 connector.

8-pin RJ45 Pin Out
Signal NamePin Number
Tx/Rx +1
Tx/Rx -2
Connector ID +4
Connector ID -6
No Connect7
No Connect8

Pins 4 and 6 are optional

Type 'A' connectors are produced by Molex and AMP, Type 'B' connectors are produced by AMP.

IPMI, Intelligent Platform Management Interface is an upper lever standard.

{ICMB Bus Index}

ICMB Interface IC Manufacturers

Refer to the RS-485 page for interface IC's.

IC Chip Manufacturers {All other functions}

{ICMB Bus Index}

IPMI Standards Info

IPMI - Intelligent Platform Management Interface Specification Second Generation V2.0, Revision 1.0 Feb 15 2006

ICMB - Intelligent Chassis Management Bus Bridge Specification V1.0, Revision 1.3 April 2 2003

The I2C-Bus Specification Version 2.1 Jan. 2000.
The I2C standard was released by Philips, which is now NXP

Editor Note: An RS485 circuit is shown but the prefix RS was only the correct reference up to the 1970's.
However the RS prefix is still used when referencing standards that were developed before 1970.
The correct reference has been EIA/TIA, however the EIA was dissolved in 2011, so the only correct specification name is TIA485.

PC motherboard

Distributor rolodex Electronic Components Electronic Equipment EDA CDROM Software Engineering Standards, BOB card Cabled Computer Bus Electronic Engineering Design Table Conversion DB9-to-DB25.
DistributorsComponents Equipment Software Standards Buses Design Reference

Modified 2/14/12
Copyright © 1998 - 2016 All rights reserved Larry Davis