Compare (input instruction)
GRT: Greater Than
Passes power while A is greater than B.
Also called: compare, greater than, >.
GRT(a, b)What it does
A compare instruction sits in the rung exactly where a contact would, and behaves like one: it either passes power or it does not. The difference is that it is testing a number rather than a bit.
Comparisons are how analog values become logic. A level transmitter gives you a number; GRT is what turns that number into a pump start.
Both sides can be a tag or a constant, which means you can compare a process value against a fixed limit or against another tag such as an operator setpoint.
A rung you can run
Start a pump on high level, stop it on low, with hysteresis
GRT(Tank_Level,800)OTL(Pump_Run)
LES(Tank_Level,200)OTU(Pump_Run)
XIC(Pump_Run)OTE(Pump_Output)Two different thresholds and a latch. The pump starts above 800 and does not stop until the level falls below 200, so a level hovering near one number cannot make it chatter.
Run this rung in the simulatorWhat it is called on other controllers
| Platform | Equivalent |
|---|---|
| Rockwell (Studio 5000) | GRT |
| Siemens (TIA Portal) | CMP > |
| Mitsubishi (GX Works) | > compare |
| CODESYS and IEC 61131-3 | GT |
Common mistakes
- Using a single comparison where the process needs hysteresis. A pump started by GRT(Level, 80) and stopped by LES(Level, 80) will chatter every time the level sits near 80. Use two different thresholds, or latch the state.
- Comparing a timer accumulator against a preset when the timer's own .DN bit already says the same thing more cheaply and more clearly.
Practise it
Interactive exercises that use GRT. Each one checks your ladder against a real scan.