Boolean logic, comparisons and bit operations

The four boolean operators

In ST the logic is words, not symbols: AND, OR, XOR, NOT.

AND is contacts in series, OR is contacts in parallel. XOR has no direct ladder equivalent, which is exactly why it earns its keep: it is TRUE only when the two signals disagree.

XOR on the job

Its most common industrial use is discrepancy detection:

SensorFault := LSH XOR (Level >= 90.0);

A high level switch and an analogue level transmitter should say the same thing. If they disagree, one of them is broken. The same line compares a valve open feedback against the open command.

Comparison operators

A comparison produces a BOOL, so you can assign it or pass it straight on:

HiAlm := Level >= 90.0;
Ready := (State = 0) AND (Faults = 0);

Never compare reals with =. Level = 50.0 may never be true once, because of floating point representation. Use a band:

AtSetpoint := ABS(Level - 50.0) < 0.5;

Bits inside a word

A status word from a VFD or a comms module packs several flags into one integer. To read one flag you use a mask: AND the word with a number that has a single bit set.

(* bit 0 ready, bit 1 running, bit 3 faulted *)
IsReady   := (StatusWord AND 1) <> 0;
IsRunning := (StatusWord AND 2) <> 0;
IsFaulted := (StatusWord AND 8) <> 0;

To set a bit: Ctrl := Ctrl OR 4;. To clear it: Ctrl := Ctrl AND 65531;. To toggle it: Ctrl := Ctrl XOR 4;.

Shift and rotate functions

The standard defines four word functions:

A shift left by one doubles the value, a shift right halves it, and both are cheaper than multiply and divide on a small CPU.

(* vendor code: this does NOT run in the lab on this site *)
Doubled  := SHL(Value, 1);
Halved   := SHR(Value, 1);
Walking  := ROL(Pattern, 1);     (* a running light across eight outputs *)
HighByte := SHR(Word16, 8) AND 255;

Note: the lab on this site supports AND, OR, XOR, NOT and MOD only. It does not support SHL, SHR, ROL or ROR. The block above is there to read and to export to your vendor's environment; the lab program in this lesson sticks to the supported operators.

Multiplying and dividing by 2 or 256 gives the same answer as a shift and does run in the lab:

Doubled  := Value * 2;
HighByte := (Word16 / 256) MOD 256;