Siemens SIMATIC Manual page 21

Temperature
Do not heat with LMN = LMN_LLM
Heat with LMN = LMN_HLM
Note
Before activating the control zone manually, make sure that the control zone
band is not too narrow. If the control zone band is too small, oscillations will occur
in the manipulated variable and process variable.
Advantage of the Control Zone
When the process value enters the control zone, the D-action causes an extremely
fast reduction of the manipulated variable. This means that the control zone is only
useful when the D-action is activated. Without a control zone, basically only the
reducing P-action would reduce the manipulated variable. The control zone leads
to faster settling without overshoot or undershoot if the output minimum or
maximum manipulated variable is a long way from the manipulated variable
required for the new operating point.
Manual Value Processing (MAN_ON, MAN)
You can switch over between manual and automatic operation. In the manual
mode, the manipulated variable is corrected to a manual value.
The integral action (INT) is set internally to LMN - LMN_P - DISV and the derivative
action (DIF) is set to 0 and synchronized internally. Switching over to automatic
mode is therefore bumpless.
Note
During tuning, the MAN_ON parameter is not effective.
PID Temperature Control
A5E00125039-02
Continuous Temperature Controller FB 58 "TCONT_CP"
Upper control zone
Lower control zone
SP_INT + CON_ZONE
SP_INT
SP_INT - CON_ZONE
Time
2-7