CPU 226 I/O and memory budget
The Siemens 6ES7216-2AD21-0XB0 is an S7-200 CPU 226 compact unit running on 24 VDC supply, packing 24 digital inputs and 16 digital outputs — 40 local I/O points that cover most standalone machine control without an expansion rack. The 8 KB code / 5 KB data memory ceiling means the application firmware and symbol table fit within a tight budget; ladder logic with extensive indirect addressing or recipe arrays will hit the data limit before the code limit. Two PPI/FREEPORT ports let the CPU talk to operator panels, drives, or a second programmer without a switch — useful when the HMI sits on one port and the commissioning laptop on the other. The FREEPORT mode supports user-defined serial protocols up to 38.4 kbaud.