Chapter Three: Overview Priorit
of Commands
29
Priority of Commands
The 649 controller has an established hierarchy that it uses to determine which commands take
precedence. The commands and operating modes are listed according to the order of priority
(from highest to lowest):
•
Valve Close Command
•
Valve Open Command
•
Set Point Recognition Operating Mode
•
Closed-Loop Control Operating Mode
Valve Close and Valve Open Commands:
The valve override commands immediately move the
valve to the appropriate position, either closed or open. These valve commands take precedence
over any other operation. The overline
(
)
indicates that the valve commands are active
when their respective pins are pulled low. For example, if the valve is currently operating under
pressure control to maintain a desired pressure, and the Valve Open pin (pin 4) is pulled low, the
valve will move to the fully open position. The Valve Close command has the highest priority.
Therefore, if both the Valve Open and Valve Close commands are issued, the valve will move to
the fully closed position.
Set Point Recognition Operating Mode:
The 649 controller can control pressure, within its
specifications, over the range from 5 to 100% of full scale. Under certain conditions the 649 is
capable of controlling below 5% down to 1%, although performance in this control range is
application dependent. If the set point signal is
less than
1% of full scale, the set point
recognition becomes effective. The 649 controller positions its valve in the fully closed position.
Increase the set point signal to a value
greater
than
1% of full scale to resume pressure control.
Closed-Loop Control Operating Mode:
When the 649 controller operates in closed-loop control
mode, it compares the pressure signal from its transducer (pin 2) or any signal on the Optional
Input pin (pin 10) to the set point. The 649 controller positions its valve to achieve or maintain
the set point pressure (or other variable if the Optional Input is used) in the system. Closed-loop
control mode has the lowest priority. The 649 controller defaults to closed-loop control in the
absence of any higher priority commands.
Refer to
I/O Connector
, page 18, for more information on the Optional Input function.
To Next Page
