Siemens SIMATIC S5 Manual page 102

Byte 28 Heating-cooling ratio as a percentage (0 to 255%)
The difference in effectiveness of the cooling and heating functions of 3-step
controllers can lead to oscillations with a normal controller action. The
heating-cooling ratio, specified as a percentage, can help prevent this, as well as
separate parameter sets for heating and cooling.
Example 1 : water cooling
10 s heating raises the temperature by 2 C. 10 s cooling lowers the temperature
by 4 C. The cooling function is approximately twice as effective as the heating
function. If 50% is entered as the heating-cooling ratio, the cooling is only activated
for half as long as the heating, i.e. only 5 s instead of 10 s.
Example 2 : air cooling
10 s heating increases the temperature by 2 C. 10 s cooling lowers the
temperature by 1 C. The cooling is approximately half as effective as the heating
heating-cooling ratio 200%.
Byte 29 Response value as a percentage (0 to 50%)
If the calculated percentage manipulated variable is less than the response value,
0 is output; if it is greater than 100% minus the response value, 100% (= sampling
time) is output (see Fig. 2.1/3).
Fig. 2.1/3 Response value
IP244
C79000–B8576–C860–02
Data Exchange with the Central Controller
4–35