CP6100 I/O Process Programming Manual

Introduction
Second Generation: Programmable Communication Subsystem (6100)
The 6100 subsystem is designed to provide the same function as
its predecessors while offering greater fault-tolerance and
flexibility. Well-planned configurations can also show
substantial improvements in throughput over configurations
based on the older hardware. The next few paragraphs describe
the features that account for these improvements.
The most obvious difference between the new subsystem and its
predecessors is that the 6100 controllers come in pairs.
(A "subsystem" consists of two controllers, hardware interfaces
for up to fifteen lines, and all the software required to control
| the hardware.) Each line can be associated not only with a
primary and backup processor, I/O process, channel, and
controller port, but also with a primary and backup controller,
as shown in Figure 1-4. Whereas with a first generation
controller, there were two paths to a line, the new subsystem
| offers up to four paths:
• A path through the primary processor and primary controller
• A path through the backup processor and backup controller
• A path through the primary processor and the backup controller
• A path through the backup processor and the primary controller
Whereas with a first generation controller, a failure could cause
the loss of 32 lines--you lost all the lines attached to a
controller if that controller failed--a failure in the new
subsystem affects at most one line. (See Figure 1-4a. If a
controller fails, there are paths to all lines through the other
controller; if a line-specific interface fails, you lose only
that line.)
| NOTE
|
| In the single-port controller configuration, there are
| only two paths to a line. Still, a single component
| failure affects at most one line. Figure 1-4b shows
| the paths through a single-port controller configuration.
An important point about the controller pairs is that not all the
lines attached to a subsystem need have the same primary
controller. You get the best performance from the subsystem if
you divide the workload between the controllers: put some lines
under primary control of one of the two controllers, and the
| other lines under primary control of the other controller. Each
| controller can be associated with a different primary processor,
| so this strategy balances the load between processors, as well as
October 1985
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