Diagnostic Conditions That Trigger BAD Signal Status on Digital Input Channel Blocks

A digital input channel block (Pcs7DiIn or F_CH_DI) asserts 'BAD = 1' (signal status 16#00) when the I/O module detects hardware failure, lost sensor supply power, wire break (on NAMUR or diagnostic-enabled channels), short-circuit to ground, or communication station loss. Unlike analog inputs where out-of-range thresholds (underflow/overflow) trip bad status, digital inputs rely strictly on electrical channel diagnostics, missing external sensor supply voltage, discrepancy time violations in redundant pairs, and upstream rack failure.

Comparison: Digital vs. Analog Channel Failure Mechanisms

In SIMATIC PCS 7 channel block architecture (Pcs7DiIn in standard APL, F_CH_DI in Safety Matrix / F-Systems), signal quality processing differs fundamentally between analog and digital inputs:
* Analog Inputs (Pcs7AnIn): Frequently assert BAD = 1 due to process threshold violations—specifically measuring current or voltage falling below 3.6 mA (underflow / live zero wire break) or exceeding 21 mA (overflow / short-circuit).
* Digital Inputs (Pcs7DiIn): Represent discrete binary states (0 or 1). A binary state cannot be "out of range." Therefore, a digital input channel block asserts BAD = 1 (and sets signal status ST = 16#00) strictly on the basis of active hardware diagnostic interrupts generated by the input module, sensor power loss, or complete communication failure.

Specific Hardware and Electrical Conditions Triggering BAD

A standard or fail-safe digital input module generates diagnostic interrupts that drive BAD = 1 under the following verified physical conditions:

Loss of Sensor Supply Power (Missing L+ / Encoder Supply Fault):
* Modern distributed I/O systems (such as ET 200SP HA and ET 200M) feature internal sensor power supplies (e.g., 24 VDC encoder supplies per channel or channel group).
* If the external 24 V power supply fails, or if the module detects an overload/fuse trip on the sensor supply output, the channel asserts BAD = 1.
Short-Circuit to Ground (M):
* In ET 200SP HA digital input modules with integrated channel power feeds, the module continuously monitors the 24 V supply line. A short-circuit to ground trips the channel electronic limiter and sets BAD = 1.
Wire Break Detection (Open Circuit):
* NAMUR Digital Inputs (Ex / Hazardous areas): NAMUR sensors operate with continuous quiescent current (e.g., 1.2 mA for OFF, 2.1 mA for ON). If the current drops below 0.35 mA, the module flags an open wire break, driving BAD = 1.
* Standard 24 VDC Inputs with Diagnostic Resistors: When enabled in HW Config, placing a shunt resistor network (e.g., 10 kΩ parallel, 1 kΩ series) across contact switches allows the module to sense quiescent bleed current. An open circuit asserts BAD = 1.
Internal Module Component Failure:
* RAM/ROM checksum errors, ADC comparator timing failures, or internal micro-controller watchdog timeouts trigger module-level fault interrupts.

Redundancy Discrepancy and Station-Level Failures

Beyond localized channel electrical faults, system-level mechanisms drive BAD = 1:

Redundant Channel Discrepancy Errors (1oo2 / 2oo3 Evaluation):
* When using redundant DI modules or fail-safe 1oo2 evaluation, if Channel A reads 1 and Channel B reads 0 for longer than the configured discrepancy time (e.g., 50 ms), the driver declares a discrepancy fault and asserts BAD = 1.
Upstream Communication / Station Failure:
* If the interface module (IM 155-6 PN or IM 153-2) drops offline, or if a backplane bus communication error occurs, OB 86/OB 82 interrupts trigger the module driver (MOD_D16_PN), which sets Bad = 1 or clamps the signal status to 16#00 / 16#68.
Module Extraction / Incorrect Module Inserted:
* Pulling the physical terminal block or module from the active backplane bus triggers OB 83, instantly forcing BAD = 1 on all associated channel instances.

Verification

In SIMATIC CFC, open the target Pcs7DiIn block in dynamic test mode.
In the field or test bench, induce an open circuit on a NAMUR channel or disconnect the 24 V sensor supply feed.
Verify that:
The channel LED on the ET 200SP HA module illuminates red (DIAG/FAULT).
In CFC, Pcs7DiIn.Bad transitions from 0 to 1.
Pcs7DiIn.PV_Out.ST drops from 16#80 (Good) to 16#00 (Bad).
Downstream logic evaluating Bad immediately executes fail-safe fallback or interlock actions.

Limitations and Operational Precautions

Standard non-diagnostic 24 V digital input modules cannot detect wire break on dry contacts without external shunt resistors. A broken wire will simply appear as a normal 0 state (de-energized) rather than BAD = 1.
Wire break diagnostics must be explicitly checked in HW Config module properties; they are often disabled by default to prevent nuisance alarms on unused channels.
In fail-safe architectures (F_CH_DI), clearing a BAD condition requires an operator or program acknowledgment (ACK_REI) after the hardware fault clears.

Engineer FAQ

Can a standard 24 V dry contact detect wire breaks?
Only if a resistor network is installed across the field switch and the module supports wire break diagnostics, or if using a specialized NAMUR digital input module.
Why did my digital input stay at 0 instead of setting BAD when the wire was cut?
The digital input module was likely a standard non-diagnostic card, or wire break diagnostics were unchecked in HW Config. The CPU simply sees 0 V and treats it as a legitimate open contact.
Does Pcs7DiIn support fail-safe substitute values?
Yes. In Pcs7DiIn, setting Feature.Bit29 enables substitute values, allowing the block to output a pre-defined Boolean state (SubsPV_In) when Bad = 1.

Technical References

SIMATIC PCS 7 Advanced Process Library (APL) Function Manual (A5E43360408-AA).
SIMATIC ET 200SP HA Digital Input Module DI 16x24VDC Manual (A5E39294170-AB).
SIMATIC S7-400 / S7-410 Diagnostic Interrupts and Channel Block Architecture.

Related Tags

PCS 7, APL, Pcs7DiIn, Digital Inputs, ET 200SP HA, Diagnostics, Wire Break, Fail-Safe

Diagnostic Conditions That Trigger BAD Signal Status on Digital Input Channel Blocks