Siemens SIMATIC S5 Manual page 81

Line break
All analog inputs with sensors directly connected (thermocouples, Pt 100s) are checked for
line break.
If a line break is detected (no actual value present), the following reactions are triggered
depending on the configuration:
control is disabled and a manipulated variable averaged over a selectable period
of time is output until the line break is dealt with (emergency program,
see Section 2.3).
or
the manipulated variable is disabled until a percentage value for the manipulated
variable is entered manually (manual manipulated variable).
or
the module switches automatically to a substitute sensor (or analog input)
connected to a different input, if inputs are still free. The substitute sensor bit is set
and
error identifiers are set (line break message A and B) and the maximum value
(460 to 3063 C) is indicated as the actual value.
Summary of the messages
Individual error bits are set separately for each controller (0 to 12) in the following situations:
– 1st positive tolerance exceeded,
– value below 1st negative tolerance,
– 2nd positive tolerance exceeded,
– value below 2nd negative tolerance,
– temperature setpoint too high,
– lower setpoint higher than temperature setpoint or too high,
– line break in thermocouple or Pt 100,
– switch over to substitute thermocouple,
– short circuit at the Pt 100,
– short circuit identified with thermocouple.
IP244
C79000–B8576–C860–02
Description of the Firmware
4–13