Siemens S5-135U/155U System Manual page 315

Digital Input
A digital input module converts the process signals to the internal signal
Modules
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.
Digital Output The control signals processed in the PLC are output via digital output
Modules
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.
BASP 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,
Signal output H+ on digital output modules for direct voltage emits a signal
Short-Circuit
when a short-circuit to ground (L-) or overcurrent has been detected at one or
Detection 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.
System Manual
C79000-G8576-C199-06
Digital Input/Output Modules
8-3