The Backplane; Message Passing Bus (Mpb) - HP M2350A Service Manual

Understanding the Computer Module
The Backplane
The Backplane
The backplane provides the connectors used to plug in the various function cards, and the
circuit board on which they are mounted. The connectors are mounted on both sides of the
backplane, dividing the computer module into two halves. The function cards may be located
either in the front of the computer module (slots 2 through 24; even-numbered connectors) or
the rear (slots 1 through 21; odd-numbered connectors).
The backplane can be split into three logical parts or busses, as follows:
Message
Passing Bus
Local Bus
This is the global communication bus for the system; used for communications
between intelligent interface cards (for example: RS232 Interface and the
Display Controllers).
This is the local communication system. It is a buffered processor bus that
allows access to local memory cards (for example: SRAM and EPROM) that
contain software for the exclusive use of the master processor.
The local bus appears on all the connectors on the backplane but is interrupted
to connect groups of connectors into cZu.sters.
Utility Bus This includes some functions that are needed for the whole system, such as
power and clock signal distribution.
Message Passing Bus (MPB)
The Message Passing Bus (MPB) is used for global system communications. Data, in the form of
messages, is passed between intelligent interface cards (for example the RS232 Interface) via
the MPB.
The MPB can be divided into the following three logical parts or busses:
Data Transfer Bus
Control Bus
Arbitration Bus
Data is written to the data transfer bus as a message. There is no address bus in the MPB
because the CPC card does not require either the destination or the source of any data. The
CPC &roadcasts messages on the bus. Within a message is a one word header that describes the
type of data and the length.
Arbitration is necessary because there is more than one participant on the bus.
Local Bus
The local bus is a typical processor bus which can be divided into the following three logical
parts or busses:
Data Bus (16-bit)
Address Bus (24-bit)
Control Bus
2-2