MKS 640B - Priority of Commands

Priorit
y
of Commands Chapter Three: Overview
38
Priority of Commands
The 640B Series 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 640B Series controller can control pressure over
the range from 2% to 100% of full-scale. If the set point signal is less than 1% of full-scale, the
set point recognition becomes effective. The 640B Series controller positions its valve in the
fully closed position for downstream control, or the fully open position for upstream control.
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 640B Series 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 640B Series 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 640B Series controller defaults to
closed-loop control in the absence of any higher priority commands.
Refer to The I/O Connector, page 13, for more information on the Optional Input function.