Siemens SIMATIC S5 Manual page 100

Byte Upper control zone (ZONOB)/ramp slope
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For zone control, bit 1 in control byte 2 = 0.
Upper control zone ZONOB: see Fig. 2.1/2.
If setpoint ramping is required; bit 1 in control byte 2 = 1.
The current setpoint is indicated in message 21 and the error messages in
message 16 relate to the setpoint in message 21.
Byte Lower control zone ZONUN (see Fig. 2.1/2)
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The upper and lower control zones are limited by the software to the maximum
possible temperature value if the value entered exceeds the control range.
Control zones (zone upper limit, zone lower limit)
Among other things, temperature controllers should meet the following two general requirements:
a) shortest possible heating time with minimum overshoot
b) compensation of temperature disturbances as quickly as possible
Requirement b) means that a relatively short integral action time is required of the PID controller.
With larger setpoint jumps (e.g. heating) this leads, however, to a fast '"saturation" of the I-branch.
The result is overshoot way beyond the setpoint.
Remedy:
Calculation of the control zone:
The control zone is determined by the characteristics of the controlled system. The important
parameters are the dead time and the maximum temperature curve of the system (see "Notes on
Settings", Chapter 4).
In general, it is sufficient to enter the control zone symmetrically, i.e. the upper limit of the control
zone = the lower limit of the control zone.
IP244
C79000–B8576–C860–02
Data Exchange with the Central Controller
4–33