Watlow Series D8 - Manual (Page 136)

Chapter 5: Tuning and Control Series D8 User’s Guide
118 Watlow Anafaze Doc. 0600-3120-2000
Control Outputs
The controller provides open collector outputs for control.
These outputs normally control the process using solid state
relays.
Open collector outputs can be configured to drive a serial dig-
ital-to-analog converter (Serial DAC) which, in turn, can pro-
vide 0 to 5 Vdc, 0 to 10 Vdc or 4 to 20 mA control signals to
operate field output devices.
Output Control Signals
The following sections explain the different control output
signals available.
On/Off
When on/off control is used, the output is on or off depending
on the difference between the set point and the process vari-
able. PID algorithms are not used with on/off control. The out-
put variable is always off or on (0 or 100 percent).
Time Proportioning (TP)
With time proportioning outputs, the PID algorithm calculates
an output between 0 and 100 percent, which is represented by
turning on an output for that percent of a fixed, user-selected
time base or cycle time.
The cycle time is the time over which the output is propor-
tioned, and it can be any value from 1 to 255 seconds. For ex-
ample, if the output is 30 percent and the cycle time is ten
seconds, then the output will be on for three seconds and off
for seven seconds. Figure 5.5 shows examples of time propor-
tioning and distributed zero crossing (DZC) waveforms.
Figure 5.5 Time Proportioning and Distribut-
ed Zero Crossing Waveforms
Time Proportioning (30%)
On
Off
0 3 10
Seconds
(Cycle Time = 10)
Distributed Zero
0 1 3 4 6
AC Cycle
Crossing (33%)