Watlow PPC-2000 - Manual (Page 252)

Chapter 6: Tuning and Control PPC-2000 User’s Guide
224 Watlow Anafaze Doc.# 30002-00 Rev 2.3
Control Outputs
The controller provides open collector and analog outputs for
control. Open collector outputs normally control the process
using solid state relays.
Open collector outputs can be configured to drive a Serial
Digital-to-Analog Converter (SDAC), that in turn, can provide
0-5Vdc, 0-10Vdc or 4-20mA control signals to operate field
output devices.
Output Control Forms
The following sections explain the various control output
signals available.
On/Off
When On/Off control is used, the output is on or off depending
on the difference between the setpoint and the process variable.
PID algorithms are not used with On/Off control. The output
variable is always off or on (0% or 100%).
Time Proportioning (TP)
With time proportioning outputs, the PID algorithm calculates
an output between 0 and 100%, 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 proportioned, and it can be any value from 1 to
255 seconds. For example, if the output is 30% and the cycle
time is 10 seconds, then the output will be on for 3 seconds and
off for 7 seconds. Figure 6.5 shows example TP and Distributed
Zero Crossing (DZC) waveforms.
Figure 6.5 Example Time Proportioning and Dis-
tributed Zero Crossing Waveforms