Watlow Electric Revision 5 - Manual (Page 55)

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Heating applications normally uses REVERSE ACTION and cooling
applications normally will use DIRECT ACTION. The selection may also
be dependent upon the application of two competing mediums of energy
such as in a HEAT/COOL or TEMPERATURE/HUMIDITY applications.
8.1.14 HEAT/COOL DUAL OUTPUTS
Certain processes such as plastic molding, plastic extrusion, refrigeration
systems, test chambers, and others require both heating and cooling control
loops. In many cases a single process variable is used for dual output, that
is, it controls both heating and cooling. To optimize the process it may be
necessary to have different setpoints with a deadband between them,
different PID or other control constants, and different output types for the
heat/cool loops. For example, a mold may be heated using PID control
through a phase angle fired power controller, while cooling is accomplished
using on/off control through a cooling water valve.
Dual outputs are provided by directing a single analog input to two control
loops. This approach is practical because of the low system cost. Two
configurations are available:
For systems with only a few heat/cool loops, a single sensor is
connected to two analog inputs. One loop is then used for heating
and the second for cooling. This is practical in the SYSTEM 32
since the cost per loop is reasonably low and fully isolated inputs are
available if needed.
For systems that are nearly all heat/cool, a single reed relay analog
input module can be jumper set to direct each analog input to two
independent loops.
In both cases each loop is fully independent and can have its own setpoint,
control mode, and output signal type. The separate setpoint allows for a
deadband adjustment not normally possible in many controllers. Three-
Position or Two-Position floating control can be implemented by selecting
the proper setpoints for the heating and cooling loops.
8.1.9 CONTROL OUTPUTS
The SYSTEM 32 provides two signal types for use as control outputs:
digital and analog. The type of output is selected from software. If the
system uses any analog outputs the optional A32-AOM ANALOG
OUTPUT MODULE is required.
8.1.10 DIGITAL OUTPUT
Digital outputs normally control the process using relays. Two types of
control are used: ON/OFF and TIME PROPORTIONING. Time
proportioning is also referred to as pulsed DC output.
On/off control has been described in section 8.1.3.