Compare (input instruction)
GEQ: Greater Than or Equal
Passes power while A is greater than or equal to B.
Also called: greater than or equal contact, compare contact, >= test.
GEQ(a, b)What it does
GEQ is a contact, not an output. It sits on the condition side of the rung with the contacts, and it passes power for as long as the comparison holds. Either side can be a tag or a fixed number, so GEQ(Tank_Level, 800) and GEQ(Tank_Level, High_Setpoint) are both normal.
The only thing separating GEQ from GRT is what happens at the boundary. Use GEQ when landing exactly on the target counts as arriving: a fill that must reach 750 units is finished at 750, and GRT would sit there waiting for a 751 that may never come. That single choice is the most common off-by-one in ladder logic.
Compares are level sensitive, not edge triggered. A GEQ that is true stays true every scan for as long as the numbers say so, which is what you want for a permissive and what catches people out when they expected it to fire once.
A rung you can run
Fill to the target, not one unit over
GEQ(Fill_Actual,Fill_Target)OTE(Fill_Done)
XIO(Fill_Done)XIC(Fill_Enable)OTE(Fill_Valve)The valve is open while the fill is not done. The instant Fill_Actual reaches Fill_Target, and not one unit later, GEQ passes power, Fill_Done comes on and the valve rung drops out. Set Fill_Target from the tag panel, then drive Fill_Actual up and watch it change over at exactly the target.
Run this rung in the simulatorWhat it is called on other controllers
| Platform | Equivalent |
|---|---|
| Rockwell (Studio 5000) | GEQ |
| Siemens (TIA Portal) | The comparator box, set to CMP >= |
| Mitsubishi (GX Works) | LD>= / AND>= / OR>= |
| CODESYS and IEC 61131-3 | GE, or >= in structured text |
Common mistakes
- Reaching for GRT when the target value itself should count. On a fill, a batch count or a position, the machine then waits for a value it may never hit exactly, and the fault looks like a stuck sensor rather than a wrong instruction.
- Comparing a raw analog value with no deadband. A noisy level sitting near the threshold crosses it repeatedly and the output chatters. Latch the result, or turn the comparison off with a second one at a slightly different value.
Practise it
Interactive exercises that use GEQ. Each one checks your ladder against a real scan.