450 Electrical Power Systems Technology
be done by laboriously writing down each mnemonic and label, line by
line, and then entering the translation into memory. A convenient alter-
native to this procedure is to have a program that takes lines of assembly
language as its input and does the translation for the programmer auto-
matically. A program called “assembler” functionally performs this op-
eration. It actually puts together the operational codes and the address-
es that take the place of mnemonics and labels. When the procedure is
complete, the assembler has a duplicate of the assembly language in the
machine language of the microprocessor. These data can then be entered
by the programmer manually, but they are generally applied directly to
the input, since it is in memory when the translation is completed.
Relay Language or Logic
The processor of a programmable logic controller dictates the
language and programming procedure to be followed by the system.
Essentially, it is capable of doing arithmetic and logic functions. It can
also store and handle data, and continuously monitor the status of its
input and output signals. The resulting output being controlled is based
on the response of the signal information being handled by the system.
The processor is generally programmed by a keyboard panel program
panel, or CRT terminal.
In a relay language system, the basic element of programming is
the relay contact. This contact may be NO or NC. Typically, each contact
has a four-digit reference number. This number is used to identify spe-
cific contacts being used in the system. The contact is then connected in
either series or parallel to form a horizontal rung of a relay ladder dia-
gram.
Once a relay program has been entered into the PLC, it may be moni-
tored on the CRT and modified by a keystroke. Monitoring the operation
of a program is achieved by illuminating the current path by making it
brighter than the remainder of the circuit components. Modification of
the diagram can be achieved by simply placing the cursor of the CRT on
the device to be altered, and making the change with a keystroke. Cursor
control is achieved by manipulation of the four arrow keys on the right
of the keyboard. Contact status can be changed or bypassed, and dif-
ferent outputs can be turned on or off, by this procedure. Each input or
output has a four-digit number that can be altered by the numerical data
entry part of the keyboard.
All the control components of a PLC are identified by a number-