Operational Power Control Systems 449
control application.
Mini-PLCs, in general, can achieve the same control functions as
the larger programmable controllers, only on a smaller basis. Mini-PLCs
are usually smaller, less expensive, simpler to use, and rather efficient,
compared with mainframe programmable controllers. Mini-units are
now beginning to make their way into the PLC field. In the future, these
systems will obviously playa greater role in the control of industrial sys-
tems.PROGRAMMING THE PLC
Instructions for the operation of a programmable logic controller
are given to the PLC through pushbuttons, a keyboard programmer,
magnetic disks, or cassette tape. Each PLC has a special set of instruc-
tions and procedures that makes it functional. How the PLC performs is
based on the design of its programming procedure. In general, PLCs can
be programmed by relay ladder diagrams or logic diagrams. These pro-
cedures can be expressed as language words or as symbolic expressions
on a CRT display. One manufacturer describes these methods of pro-
gramming as assembly language and relay language. Assembly language
is generally used by the microprocessor of the system. Relay language
is a symbolic logic system that employs the relay ladder diagram as a
method of programming. This method of programming relies heavily on
relay symbols instead of words and letter designations.Assembly Language
Assembly language is a basic instructional set that is specific to
the type of the microprocessor used in the construction of the PLC. For
one type of microprocessor, the assembly language program is a com-
bination of mnemonics and labels that the programmer uses to solve a
control problem. If the system is programmed directly with binary num-
bers, the programmer will probably use a lookup table to write the pro-
gram in assembly language. It is usually easier to keep track of loops
and variables in an assembly language. It is also easier for others to look
at the program when they are trying to see how it works. Ultimately,
the programmer must type the coded information into the computer in
binary form. This generally means that an assembly language program
needs to be translated into binary data before it can be entered. This can