Specifications

Feedback devices can be direct or reverse acting. A direct acting device outputs a signal that

increases as the process variable increases. A reverse acting device outputs a signal that

decreases as the process variable increases. The programming of Parameters 75 - FB @

MIN and 76 - FB @ MAX depend on the type of feedback device being used.

When using a direct acting transducer, Parameter 75 - FB @ MIN should be set to the value

of the process variable feedback corresponding to the minimum feedback signal (0 VDC or 4

mA), and Parameter 76 - FB @ MAX should be set to the value of the process variable

feedback corresponding to the maximum feedback signal (5 or 10 VDC, or 20 mA). See the

example below:

Example 1: A 0-100 psi transducer outputs a 4 mA signal at 0 psi and 20 mA at 100 psi.

Program Parameter 75 to 0.0 PSI, and Parameter 76 to 100.0 PSI (This assumes that

Parameter 31 - UNITS is set to pid PSI, and Parameter 33 - UNITS DP is set to XXX.X).

For a reverse acting transducer, set Parameter 75 - FB @ MIN to the maximum process

variable feedback value, and set Parameter 76 - FB @ MAX to the minimum process

variable feedback value. In other words, Parameter 75 is set higher than Parameter 76. See

the example below:

Example 2: A 0-100 psi transducer outputs a 20 mA signal at 0 psi and 4 mA at 100 psi.

Program Parameter 75 to 100.0 PSI, and Parameter 76 to 0.0 PSI (This assumes that

Parameter 31 - UNITS is set to pid PSI, and Parameter 33 - UNITS DP is

set to XXX.X).

19.2 THE SYSTEM - DIRECT AND REVERSE ACTING

The entire system can also be direct or reverse acting. In a direct acting system, an increase

in motor speed causes an increase in the process variable. This is equivalent to requiring an

increase in motor speed in response to a decreasing process variable feedback.

In a reverse acting system, an increase in motor speed causes a decrease in the process

variable. This is equivalent to requiring an increase in motor speed in response to an

increasing process variable feedback. Examples of direct and reverse acting systems are

described in the next section.