Pilz PSS Series Operating Manual page 17

Function Description
Diagnostic circuitry checks the function of the outputs. On outputs that are
switched on, the CPU will alternately switch off the "+" and "-" terminals as
part of each cycle. This is to check that the corresponding output
transistors can be switched off and that there is no interruption to the load.
In addition, outputs that are connected but switched off will be switched on
for approx. 200 µs during each self test. Where inductive loads are
switched, this time is increased. If an error occurs, the PSS will switch to a
STOP condition, switch off the outputs and trigger a message on the CPU-
display.
4-2 Operating Manual: PSS(1) DI2O Z