DESCRIPTION AND OPERATION
SYSTEM OPERATION
2 - 4
I-E96-117B
®
Analog Control Station Functions
The functions of the SAC station are to:
1. Provide process variable, set point and analog output dis-
plays (bar graph and alphanumeric).
2. Provide set point adjustment.
3. Allow M/A selection of the control loop.
4. Provide analog output backup for the CIS module in the
event of a system fault.
5. Provide raise/lower contacts for electric drive manual oper-
ation in the event of system fault.
6. Provide alarms for high/low limits or station operation
faults.
7. Provide selection options for computer control, ratio station
parameters or cascade station parameters.
The SAC station communicates to the MFP module through an
RS-485 serial link. Through this serial link, the SAC station
receives initialization and update messages from the MFP mod-
ule. Update messages contain process values to display on the
station faceplate bar graph and alphanumeric display. The
messages also contain information that directs station opera-
tion. In return, the station sends control output (in manual
operation), set point (in manual or automatic operation) and
pushbutton information to the MFP module.
The MFP module can force the SAC station into and out of
bypass mode of operation through an update message. The
station still sends the set point to the MFP module when in
bypass mode. However, the set point cannot be changed or dis-
played in bypass mode. The operator controls the demand out-
put through pushbuttons on the SAC station faceplate (open
loop control). During bypass operation, the MFP module tar-
gets the control output it normally sends to the CIS module to
match the demand output from the SAC station. Targeting the
control output prevents a significant disturbance in the control
output when the MFP module resumes control through the CIS
module.
The SAC station, when in manual override mode, responds to
MFP module update messages differently than when in bypass
mode. When the station is in manual override mode, it ignores
any MFP module update request to exit into any other mode of
operation. The station only responds to an update message
operating mode transfer request after the operator forces the
