Siemens S5-155U System Manual page 303

Digital input modules
Digital output modules
BASP (output inhibit)
Signal output,
short-circuit detection
System Manual
C79000-B8576-C199-03
A digital input module converts the process signals to the internal
signal level in the module. Interference is suppressed by the input
circuitry, and the logic states at the inputs are indicated with LEDs on
the front strip of the module. With most digital input modules (except
the 420), the signals are isolated from the central ground point when
received. Inputs relating to the same group are isolated from inputs of
another group, but not from each other.
The control signals processed in the PLC are output via digital output
modules at a signal level suitable for the actuator, e.g. contactor,
solenoid valve, etc. For the voltages and currents required in the
process, modules with appropriate output circuitry are available.
With most digital output modules (except the 441), the signals are
isolated from the central ground point when relayed to the outputs.
Outputs relating to the same group are isolated from outputs of
another group, but not from each other.
If a digital output module receives a "command output inhibit"
(BASP, cf. Programming Guide and Chapter 4) from the CPU, the
outputs are set to zero.
When the supply voltage of the central controller or expansion unit
has been switched on or when the CPU is at STOP, the outputs are set
to zero.
Signal output H+ on digital output modules for direct voltage emits a
signal when a short-circuit to ground (L-) or overcurrent has been
detected at one or more outputs currently at logic 1. Only
short-circuits which are effective for more than 0.5 to 1 s are detected.
Short-circuits are only detected at connected outputs. The signal
outputs are decoupled by diodes. Up to 16 outputs can be connected in
parallel. Ensure that isolation is not defeated by the parallel
connection of outputs.
To ensure functioning of the signal output, you must connect terminal
1L+ to 24 V on the 441, 451 and 454 modules. On the 453 and 457
modules, the floating signal output must be powered separately.
Technical Description
8 - 5
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