Siemens SIMATIC S7-1500 System Manual page 135

Configuring
7.9 Configuration of digital inputs/outputs (X142)
Reading back the terminal state
STS_DI (offset byte 0 of the feedback interface) represents the logical channel status, taking
account of any inversion configured.
Digital inputs
With digital inputs (DI, Timer DI, oversampling DI, event/period measurement), the value
corresponds to the logical state of the digital input.
Digital outputs
With digital outputs (DQ, Timer DQ, oversampling DQ, pulse width modulation PWM), the
value corresponds to the actual terminal state of the digital output. If the terminal state
deviates from the controlled state, there may be an output driver short-circuit or defect.
Note
STS_DI
Signals are only reliably acquired over STS_DI if the level is significantly longer than the
input delay + acquisition cycle of the digital inputs/outputs (X142).
Example: If you operate the digital inputs/outputs (X142) as isochronous to the MC Servo in
a cycle of 2 ms and a set input delay of 125 µs, the level duration must be > 2.125 ms.
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SIMATIC Drive Controller
System Manual, 11/2019, A5E46600094-AA