PID loop in a S7-300 rack
The Siemens 6ES7355-0VH00-0AE0 is a Control module FM 355 C PID Control — a four-channel continuous PID controller that handles closed-loop process control independently of the CPU scan cycle.
What the FM 355 C does in the loop
The FM 355 C handles four analog inputs and four analog outputs for continuous PID control — temperature, pressure, flow loops — with configurable controller type (P, PI, PID). It communicates with the CPU over the backplane bus, but executes the control algorithm locally, so loop response is not gated by the CPU program cycle.