Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 11)

Introduction
Chapter 1
1Ć8
The last level of fault tolerance results from powering the module from
external +5V DC and + l5V DC power supplies, independent of I/O
chassis backplane power, through the field wiring arm.
Application in Control Systems
The PID module performs closed loop control. Once programmed, it can
operate independently of the PC processor. System status and alarm
information can be reported to the PC processor to ensure safe system
performance.
Redundant control is another alternative. PID modules could serve as
back-up to a PC processor performing the PID loop algorithms. The PC
processor can control many complex and interaction loops using PID
modules for analog inputs and outputs. The complex algorithm could be
subdivided into individual loop algorithms stored in the PID modules
operating as analog I/O modules. In the event of a PC processor failure or
loss of communication between PC processor and a PID module, the soft
fault mode of the PID module would allow it to automatically control its
loops according to the PID loop algorithms stored in its memory if the
appropriate soft fault response had been selected.