CPU 221 for compact standalone control
The Siemens SIMATIC S7-200 CPU 221 (6ES7211-0BA20-0XB0) is the smallest fixed-I/O controller in the S7-200 family, built for standalone applications that don't need expansion modules. It carries 2 kWords of program memory and combines AC power supply with DC inputs and relay outputs — a common configuration for valve control, conveyor interlocks, and simple machine logic where the load voltage differs from the control voltage.
Program memory and I/O mix
2 kWords program memory limits the ladder logic footprint — this CPU suits applications under about 100–150 rungs of typical bit-logic and timers, not data-heavy motion or recipe management. The AC/DC/Relay designation means the CPU is powered by 120–240 VAC, the inputs are 24 VDC sourcing, and the outputs are relay contacts rated for up to 2 A per channel — the relay bank handles mixed voltages on the load side without separate interposing relays.
Listed as current production — the CPU 221 remains an active order code within the S7-200 portfolio.