The two unused analog inputs on a SIMATIC S7-1200 CPU 1214C DC/DC/DC cannot be reassigned as digital inputs. The base CPU I/O allocation is fixed; obtain additional digital input hardware or revise the external signal arrangement.
Why the analog inputs cannot become digital inputs
Configuration cannot change the onboard AI channels into DI channels. This is a hardware allocation constraint, not a missing programming option. The evidence does not identify a firmware-dependent exception or a configuration parameter that changes the channel type.
Supported alternatives
| Approach | Use case | Constraint |
|---|---|---|
| Add a signal board | Additional digital inputs are required | Select a compatible board from current Siemens documentation; no part number is established by the evidence. |
| Reassign wiring through an external relay | An existing DI can be shared by changing the machine sequence or contact arrangement | Relay operation adds response delay. |
| Multiplex sources through a double-pole relay | Two sources can use one DI at different machine states | The PLC must know which source is selected, and the slower response must suit the process. |
| Interpret an analog voltage as Boolean | A slow-changing status must be detected without another DI | This is a workaround, not DI reconfiguration; the external circuit must reduce a 24 V ON signal to 10 V or below. |
Analog-threshold workaround
A resistor divider can reduce the external ON voltage before it reaches an analog input. PLC logic can then compare the measured analog value with a chosen low/high threshold:
#myNewInput := "inAnalog1" > #LOW_ANALOG_VALUE;Do not select resistor values or a threshold from the available evidence: it provides neither analog-input electrical specifications nor source characteristics. Verify those limits in the applicable Siemens hardware documentation and account for tolerances. Do not use this method when the required signal changes faster than the analog acquisition and program response can reliably detect.
Implementation and verification
- Confirm whether the application needs an independent DI, can share an existing DI through a relay, or can tolerate analog threshold detection.
- For an independent input, install a compatible signal board and map the added channel in the project.
- For relay sharing, verify the selected contact routes only the intended source and that relay delay is acceptable.
- For analog threshold detection, measure the analog-terminal voltage in both signal states, confirm the ON state remains at or below 10 V, and verify stable Boolean behavior around the selected threshold.
FAQ
Can the S7-1200 CPU 1214C analog inputs be configured as digital inputs?
No. The onboard analog and digital channel allocation on the base CPU cannot be reassigned through configuration.
How can I add two digital inputs to a CPU 1214C DC/DC/DC?
Add a compatible signal board when independent digital inputs are required. Confirm the exact board and channel compatibility in the applicable Siemens documentation because the evidence provides no part number.
Can a 24 V digital signal be read through an S7-1200 analog input?
Only as a workaround: reduce the 24 V ON signal to 10 V or below and compare the analog value against a validated threshold. Verify electrical limits and timing because this does not provide normal digital-input switching performance.