CPU for the panel that needs to talk and count
The Siemens 6ES7216-2BF22-0XB0 is a S7-200 CPU that runs on 24V DC — keep it between 20.4 and 28.8V or it'll drop out. It's got FBD, LAD, and STL programming baked in, plus MPI and PPI on the RS485 port for talking to HMIs or other S7-200s. Six high-speed counters handle up to 30 kHz each for encoder pulses or fast flow totals. This is the one you spec when you need a standalone controller with decent I/O and a fieldbus hookup.
Backup that keeps the program alive
The integral high-performance capacitor holds the program for 190 hours at 40°C (min. 120 h). If a shutdown runs longer, the optional battery module gives you 200 days typical. That's enough for a long weekend or a plant-wide outage without reloading.