For the SMH2010 behavior addressed here, an electrically open discrete input appears as logic 1 in the FBD program. The input is therefore active-low from the program’s point of view if closing the field contact produces the complementary state. Verify that second state at the controller before treating it as a project-wide rule.
Current threshold and logical state
An open contact interrupts the intended field-current path, but the input electronics still have an internal bias and a defined logical interpretation. The CPU does not evaluate the mechanical position of the contact; it evaluates the electrical condition produced at the input circuit and then maps that condition to a Boolean value.
The number that matters is the input current or voltage relative to the input stage’s switching thresholds. Those thresholds, the input circuit topology, and any logical inversion determine the bit presented to FBD. For this SMH2010 case, the resulting open-circuit value is 1. This is threshold behavior, not an FBD execution fault.
Contact bounce and the input filter also affect timing. A transition may not become visible during the exact scan in which the contact moves. Read the configured filter or hardware documentation before diagnosing a short delay as incorrect polarity. Input loading can also produce heat in field components, but an open contact carries no intended load current; investigate unexpected current as leakage, wiring, or interface behavior.
Polarity-handling approaches
| Approach | Benefit | Risk | Best use |
|---|---|---|---|
| Use the raw input directly | No additional logic | Every network must remember that open reads 1; mixed assumptions create inverted commands and alarms |
Small, fully documented programs with one or two references |
| Invert the state wherever it is consumed | Each network can express the required local meaning | Repeated inversion is difficult to audit and may become inconsistent after modifications | Signals whose meaning legitimately differs between functions |
| Normalize the input once | Downstream logic uses one clear process meaning | The mapping layer must be documented and tested | Preferred for interlocks, permissives, alarms, and reusable function blocks |
Normalize the signal once near the input-mapping boundary. Define the internal Boolean by process meaning—such as device active, permissive healthy, or fault present—rather than by terminal voltage or contact position. When the required process state is opposite the raw bit, implement logical process state = NOT(raw input state).
Polarity normalization does not determine fail-safe behavior by itself. The contact type, field power, broken-wire response, and control objective decide whether an open circuit should command a trip, alarm, or inactive state.
Electrical and online diagnostic checks
| Quantity or state | Expected observation | Where to read it |
|---|---|---|
| Contact open | Raw SMH2010 input displays 1
|
Online input image, variable watch, or FBD monitoring view |
| Contact closed | Record the actual raw value; a normal complementary transition would produce 0
|
Same online view used for the open test |
| Terminal voltage | A stable value corresponding to each contact state | Measure at the input common and signal terminal; compare with the hardware documentation |
| Input current | On-state and off-state current on opposite sides of the specified thresholds | Read the threshold table in the SMH2010 hardware documentation and measure only when the circuit permits it |
| Transition delay | State changes after the input filter and scan processing | Input-filter configuration, online trace, or repeated watch-table observation |
Start at the raw input image, before any inversion block, latch, timer, or alarm logic. If the terminal indication changes but the raw variable does not, inspect channel mapping and hardware status. If the raw variable changes but the process variable does not, trace the FBD path through inversions, gates, memory elements, and task execution.
A voltage measurement alone may mislead when sensors or interface modules leak current in their off state. Compare both voltage and current with the documented switching thresholds. A floating conductor, missing common, or high-resistance connection can place the input near a threshold and cause an unstable value.
FBD normalization procedure
- Identify the exact raw variable mapped to the physical SMH2010 input. Confirm the channel by operating only the associated field contact while monitoring the raw variable online.
- Open the contact and record the raw value. It should display
1for the behavior covered here. - Close the contact and record the raw value. Confirm that it changes rather than assuming the complementary state.
- Write a two-row truth table that maps each verified raw state to the required process meaning. Include the intended response to loss of field power or an open wire.
- If process-active must be true when the raw input is
0, place one inversion at the input-mapping layer. Bind downstream FBD logic to the normalized project variable rather than repeatedly referencing the raw channel. - Update signal comments and electrical documentation to state that the raw open-circuit value is
1and to explain the normalized meaning. - Download the change under the site’s change-control procedure, then test both stable states and each transition.
Verification matrix
| Test condition | Raw input | Normalized state | Required check |
|---|---|---|---|
| Contact open | 1 |
Defined by the truth table | Verify commands, permissives, alarms, and displayed status |
| Contact closed | Measured during commissioning | Defined by the truth table | Verify the opposite operating condition |
| Open-to-closed transition | Changes after input processing | Changes once without unintended latching | Check filter, timers, and scan-dependent logic |
| Closed-to-open transition | Returns to 1
|
Returns to the specified process state | Check trip and alarm response |
| Loss-of-signal simulation | Measure rather than predict | Must match the risk assessment | Verify broken-wire or power-loss behavior where the circuit design permits the test |
Monitor the raw and normalized variables simultaneously. The inversion layer passes verification only when every physical state produces the intended command, indication, alarm, and interlock result. Reset retained states between tests so a latch does not conceal the current input value.
Recurring polarity pitfalls
A graphical contact symbol in FBD describes a Boolean test; it does not prove whether the field contact is physically open or closed. Confusing those two meanings is the most common source of reversed behavior.
Another failure mode is changing both wiring and program polarity during one intervention. The two inversions can cancel, leaving the normal operating test apparently correct while fault and power-loss cases remain wrong. Change one layer at a time and repeat the complete matrix.
Online forcing can also conceal wiring faults. Remove all forces after the controlled test and verify the raw value from the real terminal condition. When several channels disagree, compare channel mapping, common wiring, interface devices, and input configuration before copying an inversion across the program.
FAQ
How do I read an open SMH2010 digital input in FBD?
Treat the raw open-circuit state as logic 1. Monitor the raw mapped variable online before any inversion, timer, or latch.
How do I make the SMH2010 input true when the device operates?
Test both physical contact states, build a truth table, and invert the raw bit once if the required process meaning is opposite. Use the normalized variable throughout the downstream logic.
How do I confirm that the input is really active-low?
Record the raw value with the contact open and closed. Open should read 1 here; confirm that closing the contact produces the complementary transition at the actual controller.
How do I troubleshoot an SMH2010 input that never changes?
Compare terminal indication, measured voltage or current, raw input image, channel mapping, and hardware status in that order. Then trace the FBD path for inversion, forcing, latching, or a task that is not executing.
How do I know when to stop testing and contact support?
Stop field testing if the input is unstable near its threshold, the terminal measurements conflict with the hardware documentation, or the channel status indicates a hardware problem. Escalate to the official SMH2010 support channel with the controller identification, wiring diagram, measured terminal values, channel mapping, and open/closed online observations.