Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 10)

Introduction
Chapter 1
1Ć7
PC processor operation is changed from run mode to program or
test mode
a PC processor fault occurs
a communications cable break occurs between the PC processor
and the I/O chassis
Each loop can be programmed independently to one of the following
responses when a soft fault occurs:
set the analog output to minimum value (+4mA or +1V DC)
holds the analog output to the last value before the soft fault occurre
performs PID control based on the last values transferred to the module
before the soft fault occurred.
sets the analog output to maximum value (+20MA or +5V DC)
The second level of fault tolerance is how the module sets its outputs in
response to a hard fault. A hard fault occurs when the module detects a
failure of its microprocessor or its self-diagnostics. Internal programming
plugs are used to select the manner in which the PID module can respond
if a hard fault should occur:
sets analog output to minimum value (+4MA or +1V DC)
holds analog outputs at the last value before the fault occurred
sets analog outputs to maximum value (+20mA or +5V DC)
Note: These hard fault responses cannot be ensured if a component in the
analog output circuit should fail.
A manual control station can be connected between the PID module
analog output and the controlled element of the process. Control of the
output can be switched manually to the manual control station by a switch
at the station. Manual control at the station overrides the PID module’s
output.
The third level of fault tolerance is automatic switching of control to the
manual control station when a hard fault occurs. A relay contact in the
PID module closes automatically (manual request), putting the station in
control. The module’s output is overridden by the station output.
The fourth level of fault tolerance is the module’s response to loss of +5V
DC. Outputs respond as if a hard fault occurred (a or C above, but not b).
The same programming plug configuration selects the module’s response.
If +15V DC is lost, analog outputs go to minimum value, regardless.