Siemens SIMATIC S5 Manual page 77

The calculated analog manipulated variables as percentages are output in message 18 for
channels 0 to 12. In the programmable controller, they can be passed on to an analog output.
Percentage output
With percentage output, the average of the manipulated variable is controlled by the pulse duration
modulation at a constant frequency (= 1/T
The stronger or weaker control action of cooling compared with heating (e.g. water cooling) can be
taken into account with 3-step controllers by setting a heating-cooling ratio (as a percentage). If
manual operation is intended, the manual manipulated variable to be set as a percentage is
calculated as the corresponding proportion of the sampling time according to the pulse duration
modulation.
Heating switch (digital input)
The heating switch (socket connector X4, pin 1) can be used to disable the controller outputs, if this
was set for each controller individually.
+
Pin 1, X4 connected to L
Pin 1 open
The disabling effect of the heating switch can be cancelled for each controller by setting bit 2 in
control byte 2 of the controller messages.
When the DQs are switched off, the controllers are stopped, the control algorithm is interrupted and
error messages cleared. This allows a "bumpless" restart.
IP244
C79000–B8576–C860–02
).
A
DQs enabled;
DQs disabled.
Description of the Firmware
4–9