ON/OFF Control system is a commonly used method to control temperature. Comparison instructions, that are Less than or equal and Greater than or equal are set to control the temperature range.
A Less than or equal logic energize when the input value is less than or equal to the setpoint.
A Greater than or equal logic energize when the input value is greater than or equal to the setpoint.
Ladder Diagram for ON/OFF temperature control
S1 connected in the first line and the last line of the above ladder diagram is programmed to the heater output circuit to control the circuit.
MOV instruction carries and displays the output values of the temperature measuring device, MOV instruction addresses a thermocouple here.
All the MOV, LEQ, GEQ are addressed to the same integer file.
Source A in both LEQ and GEQ are the thermocouple current output, which is compared with the setpoint value at the source B.
Setpoint for LEQ will be the lower range value and for GEQ it is the upper range value.
Now, look at the ladder diagram above.

If source A < source B of the LEQ logic, then the low temp and heater output becomes TRUE.
The heater remains to ON as long as this two condition satisfies  Low temp output is TRUE and High temp output is FALSE. 
Even if the Low temp output turns FALSE the heating continues until the High temp turns TRUE.
Look at the latch in the last line of the ladder diagram. 
The heater continue to be ON until the temperature reaches the upper range value.

When the temperature reaches higher range value, the source A will be equal to source B of the GEQ logic. Then the High temp becomes TRUE.

The heater turns OFF when the High temp becomes TRUE.

The heater remains OFF until the temperature reaches the lower range value. And starts again.
The cycle is repeated to maintain an average setpoint temperature.