The output at address 0500 energizes only when the rung evaluates true; the register bits show the stored input and output states used to reach that result. In the example, input 0000 is logic 0, input 0001 is logic 1, and output 0500 is logic 0. The resolving condition is SW1 = 1 and SW2 = 0, provided the rung instructions evaluate true for those states.
Which register values does the rung use?
Trace the logic from the input addresses to the output coil. A physical switch changes an input circuit; the PLC represents that input as a bit in its data area. The ladder instructions then test bit states, and the output coil instruction determines the resulting output state. These are related but distinct layers: physical device state, stored input bit, instruction truth, and physical output.
| Address | Register/bit shown | Example state | Meaning |
|---|---|---|---|
0000 |
Register 00, bit 00
|
0 | Input instruction state is false when it tests this bit as a standard true-for-1 condition. |
0001 |
Register 00, bit 01
|
1 | Input instruction state is true when it tests this bit as a standard true-for-1 condition. |
0500 |
Register 05, bit 00
|
0 | Output coil state is false in the displayed snapshot. |
Read the PLC's actual data table or online monitor rather than infer bit values solely from a switch's physical position. The example reports the register mapping and values, but the ladder drawing is needed to confirm the exact instruction type and rung arrangement.
Does a physical switch position equal its input bit?
Take two readings: observe the switch or contact physically, then inspect the corresponding PLC input bit. If the bit changes as the device actuates, the signal has reached the PLC input image. If it does not, troubleshoot the input circuit, wiring, module indication, and address assignment before analyzing the rung.
A normally open or normally closed device describes its electrical contact behavior; it does not, by itself, define whether a ladder instruction is true. A ladder examine-if-closed instruction is ordinarily true when its addressed bit is 1, while an examine-if-open instruction is ordinarily true when the bit is 0. Thus an input bit at 1 can make a normally closed ladder instruction false. The evidence's requirement that SW2 be logic 0 is consistent with such an inverted test, but verify the actual instruction symbol in the program.
Where does the rung first evaluate false?
Monitor the rung online and compare each instruction's displayed truth state with its addressed bit. If an input bit is wrong, remain at the field-to-input path. If the bit is correct but its ladder instruction is false, check whether the instruction tests for 1 or 0 and whether the rung uses series or parallel logic. A false series condition blocks the path to the output coil; a parallel branch can provide another true path if the program includes one.
In the stated example, the conditions required for the coil are SW1 logic 1 and SW2 logic 0. The initial snapshot has SW1's corresponding input at 0, so the output remains false regardless of whether the other condition is satisfied. Change or simulate only under an approved operating condition, then observe the rung evaluation rather than assuming the coil follows the physical switch directly.
What should the address and register checks show?
Compare the address displayed at each ladder instruction with the module channel or assigned I/O point. Then read the bit in the register table and the coil state. PLC families differ in how they organize registers and address I/O; the example's 16-bit-register description is not a universal addressing rule. Use the controller's data table and programming documentation for the installed system.
| Check | Expected evidence | If it differs |
|---|---|---|
| Input address | The instruction address matches the intended input point. | Correct the address mapping or examine the intended point. |
| Input bit | The stored 0/1 state changes with the input circuit. | Check device, wiring, module channel, and input status. |
| Instruction truth | The rung instruction matches its 1/0 test and current bit. | Inspect contact type and branch logic. |
| Output coil | The coil state follows the complete rung evaluation. | Trace the first false condition and confirm output address. |
How do you correct and verify the output condition?
- Confirm the intended input points and addresses in the controller's I/O configuration and online data table.
- Operate each input through its expected state and confirm that its addressed bit changes. Resolve any mismatch before editing logic.
- Inspect each rung instruction and determine whether it is true for bit 1 or bit 0. Confirm the rung topology and required conditions against the intended control behavior.
- With the machine in an approved test state, satisfy the example's required conditions: SW1 logic 1 and SW2 logic 0. Observe the rung and output coil at
0500. - Confirm the physical output point changes with the coil state. If the coil is true but the field device does not respond, troubleshoot the output channel and external circuit rather than changing input logic.
Verification passes when the intended input bits are correct, every required rung instruction evaluates true, the coil at 0500 is true, and the corresponding physical output responds. Restore normal operating conditions and confirm the rung returns to its expected state.
PLC register and ladder logic FAQs
What happens if an input register bit is 0?
An instruction testing for bit 1 is false, while an instruction testing for bit 0 is true. Check the instruction type before deciding how that bit affects the rung.
What happens if one series instruction is false?
The series path to the output coil evaluates false, so the coil does not energize unless another programmed branch provides a true path.
What happens if the switch is physically ON but its bit is 0?
The field signal is not represented as expected at that address. Check contact wiring, input module status, channel assignment, and the monitored address.
What happens if the output coil is true but the device stays off?
The logic has reached the coil, so inspect the output address, module channel indication, external wiring, and load circuit.
What should I verify last before returning the PLC to operation?
Verify the input bits, rung instruction states, coil at 0500, and physical output response in sequence; then confirm the rung returns to its expected state under normal inputs.