12.4.4Data comparison instructions

As we have seen with counter and timers, some PLC instructions generate digital values other than simple Boolean (on/o ) signals. Counters have current value (CV) registers and timers have elapsed time (ET) registers, both of which are typically integer number values. Many other PLC instructions are designed to receive and manipulate non-Boolean values such as these to perform useful control functions.

The IEC 61131-3 standard specifies a variety of data comparison instructions for comparing two non-Boolean values, and generating Boolean outputs. The basic comparative operations of “less than” (<), “greater than” (>), “less than or equal to” (≤), “greater than or equal to” (≥), “equal to” (=), and “not equal to” (=)6 may be found as a series of “box” instructions in the IEC standard:

The Q output for each instruction “box” activates whenever the evaluated comparison function is “true” and the enable input (EN) is active. If the enable input remains active but the comparison function is false, the Q output de-activates. If the enable input de-de-activates, the Q output retains its last state.

Motor B runtime. These two integer values are input to the “greater than” instruction box for comparison. If motor A has run longer than motor B, motor B will be the one enabled to start up next time the “start” switch is pressed. If motor A has run less time or the same amount of time as motor B (the scenario shown by the blue-highlighted status indications), motor A will be the one enabled to start. The two series-connected virtual contacts OUT motor A and OUT motor B ensure the comparison between motor run times is not made until both motors are stopped. If the comparison were continually made, a situation might arise where both motors would start if someone happened to press the Start pushbutton with one motor is already running.

An example showing the use of such instructions is shown here, converting a temperature measurement in units of degrees Fahrenheit to units of degrees Celsius. In this particular case, the program inputs a temperature measurement of 138 oF and calculates the equivalent temperature of 58.89 oC:

PLC program

Note how two separate math instructions were required to perform this simple calculation, as well as a dedicated variable (X) used to store the intermediate calculation between the subtraction and the division “boxes.”

Although not specified in the IEC 61131-3 standard, many programmable logic controllers support Ladder Diagram math instructions allowing the direct entry of arbitrary equations. Rockwell

(Allen-Bradley) Logix5000 programming, for example, has the “Compute” (CPT) function, which allows any typed expression to be computed in a single instruction as opposed to using several dedicated math instructions such as “Add,” “Subtract,” etc. General-purpose math instructions dramatically shorten the length of a ladder program compared to the use of dedicated math instructions for any applications requiring non-trivial calculations.

For example, the same Fahrenheit-to-Celsius temperature conversion program implemented in Logix5000 programming only requires a single math instruction and no declarations of intermediate variables:

Rockwell Logix5000 PLC program