Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 31)

Assembly and Installation
Chapter 2
2Ć18
allows the PID output to be switched open at the manual control station
and a manually controlled output to be switched into the system.
When in manual mode, care should be taken not to return to automatic
control when the output is below +1V dc or +4mA. If attempted, an
output surge could occur. The PID module performs a bumpless transfer
when the tieback input is tracking a signal at or above the minimum level
of +1V dc or +4mA. Also, the PID module’s operating mode must
include integral control and the manual mode status line must be closed at
the manual control station. The module monitors tieback inputs only
when you enable manual control.
Module BackĆup
The manual control station can be used to automatically backup the PID
module in the event of module failure. A module failure would
de-energize the PID module’s contact output and generate a manual
request to the manual control station causing control to be transferred in
the following manner.
The contact output (Figure 2.9) is a normally closed contact. When the
PID module is in normal operation, the contact will be held open. A
manual request is generated by contact closure. The contact will close
and remain closed in the event of module failure or loss of +15V dc
analog power. Contact closure internally switches the signal at terminal 4
MANUAL REQUEST to terminal 11 MODULE COMMON. There is
only one contact output. If two manual control stations are used, both will
be switched simultaneously.
If a manual request is generated by program logic, contact closure is a
one-shot 50 millesecond closure. The closure can be used to activate an
alarm device by connecting a latching relay to terminal 4 MANUAL
REQUEST and terminal 11 MODULE COMMON on the PID module.
Once the PID module relinquishes control to the manual control station,
the plant operator must physically reset the operation to automatic mode
using a switch on the manual control station.
When control is returned from manual to automatic at the manual control
station, the PID module performs a bumpless transfer if the tieback analog
input is used to track the output from the manual control station and the
PID module’s operating mode includes integral control.