Page 689 - Mechanical Engineers' Handbook (Volume 2)

680 General-Purpose Control Devices

This capability may not be required if all interface to the controlled operation is
through the closed-loop controllers described above.
• Receiving commands from and sending information to a control panel or console for
the operator of the machine or process.
• Receiving commands from and sending status information to a manual or automated
system with the responsibility of supervising the operation of a number of GPCDs
within the boundaries of a ‘‘cell,’’ for example, a number of coordinated machines or
unit operations, or over a wider ‘‘area,’’ such as a chemical process or production zone
in a factory.
• Interchanging status information with other GPCDs within the same cell or across cell
boundaries.
It should be noted that not all of these capabilities are necessarily required for every appli-
cation of a GPCD. For instance, special closed-loop controllers may not be necessary in an
operation requiring only simple on–off or modulating control. Similarly, communication in
an automation hierarchy may not be required for simple ‘‘stand-alone’’ applications such as
an industrial trash compactor.
However, the capability for expansion into a communicating hierarchy should be inher-
ent in the GPCD architecture if retrofit of stand-alone systems into an integrated production
system is considered a future possibility.

1.2 Programmability
To be truly general purpose, a GPCD must be programmable; that is, its operation is con-
trolled by sequences of instructions and data stored in internal memory. The languages used
for programming GPCDs are usually problem oriented: Programs are expressed in terms
directly related to the control to be performed, rather than in a general-purpose programming
language such as C or BASIC. These languages will be described in appropriate sections
for each type of GPCD.
Depending on the application, the responsibility for development and maintenance of
GPCD programs may reside with:
• The original equipment manufacturer (OEM) of a machine that includes a GPCD as
part of its control apparatus
• The system integrator who designs and installs an integrated hierarchical control sys-
tem
• The end user who wishes to modify the operation of the installed system

The degree to which the operation of the system can be modified by the end user is a function
of:
• The complexity of the system
• The degree to which the end user has been trained in the programming of the system
• The extent to which the operation of the process must be modified over time
For instance, in a high-volume chemical process, only minor modifications of set points may
be required over the life of the plant. However, major modifications of the process may be
required annually in an automotive assembly plant. In the latter case, complete user pro-
grammability of the system is required.