Advanced Instructions Found in Larger PLCs

In larger PLCs a wide range of advanced instructions is provided. These instructions include the complete set of math functions (addition, subtraction, multiplication, division, square root, and trigonometry functions). The numbers used in these applications can also have several different formats such as integers (whole numbers), floating-point decimals (numbers with decimal points), exponential numbers, binary, hexadecimal, and binary coded decimal. Each of the number formats can be converted to other formats. Another set of functions provided in these PLCs is the compare functions, including equal, less than, greater than, and all of the complements to these.

The larger PLCs also provide a wide range of file functions. A file is a group of memory locations used to store numbers. The functions include loading a file, unloading a file, moving all addresses from one file to another, word-shift registers, word-FIFO registers, and file logic instructions. Logic instructions, for example, allow all of the memory locations of one file to be ANDed with all of the memory functions of a second file. All of the other logic functions such as NOT, OR, NAND, NOR, and EXOR are also available.

Larger PLCs also provide analog capabilities for analog input and analog output modules. The analog modules are AID or DIA modules that can convert voltages or currents to digital numbers or vice versa. The digital values can completely interface with all of the instructions inside the PLC and they can take advantage of closed-loop control functions. The closed-loop control functions in the PLC are provided as a menu where the setpoint, process variable, and output are entered as addresses and the range for each is set. The menu also accepts values for proportional gain, integral, and derivative (PID) values that will set the rate of reaction for the loop. All of the analog input and output signals are scaled to match the process variable in the system.

The important part of these larger systems is that they provide all of the machine language functions of the microprocessor, and they provide the original capability of the controlling input and output signals directly. In fact many of these PLCs cost less than $2000 and they provide the same functions as many minicomputers.