Page 691 - Mechanical Engineers' Handbook (Volume 2)

682 General-Purpose Control Devices

• The I/O (input/output) processor provides the means by which the control processor
can issue commands to and receive status information from the closed-loop controllers
as well as interchange information directly with the controlled operation via the output
and input interfaces.
• The system bus provides for communication among the functional blocks internal to
the GPCD, while the I/O bus provides for communication between the internal func-
tional blocks and the ‘‘outside world’’ via the closed-loop controllers and I/O inter-
faces. As an option, the I/O controller may extend the I/O bus functionality to remote
locations using data communications methods such as those described in Section 9.

The architecture shown in Fig. 3 is not the only one possible for GPCDs, nor is it
necessarily the most desirable for all applications. For instance:

• Large systems may require multiple control processors on the system bus.
• In applications requiring only a few control loops, it may be more economical to
perform the closed-loop control function directly in the software of the control proc-
essor.
• A separate I/O processor may not be required if the number of separate I/O interface
points is less than a few dozen.
• In small systems, the programmer’s console may be interfaced directly to the system
or I/O bus.
However, if it is anticipated that control system requirements will grow substantially in the
future or if total control system requirements are only partially understood, the use of a
flexible, extendable GPCD architecture such as that shown in Fig. 3 is recommended.

1.4 Sequential Control
It is obvious that GPCDs must perform complex sequences of control actions when they are
applied to the coordination of material handling and machine operation in the fabrication
and assembly of discrete parts or in batch and semibatch processes such as blast furnace
operation and pharmaceutical manufacture. However, sequential control is also increasing in
importance in ‘‘continuous’’ process control, since no process is truly continuous. At the
very least, the process must be started up and shut down for maintenance or emergencies
by a predetermined sequence of control actions. In large, integrated processes, these se-
quences are too complicated to be carried out manually and must be performed automatically
by GPCDs.
The increasing importance of sequential control, coupled with the increasing complexity
of the controlled processes, have generated the need for graphical programming and docu-
mentation techniques for the representation of large, complex sequential control plans. These
plans must provide a straightforward representation of the relationship between the operation
of the control program in the GPCD and the operation of the controlled machine or process
as well as the interrelationships between multiple, simultaneous control sequences.
1
Recognizing this need, the International Electrotechnical Commission (IEC) has un-
dertaken several efforts to standardize the representation of sequential control plans:
2
• The IEC 60848 standard defines the GRAFCET specification language for the
functional description of the behavior of the sequential part of a control system.