The 6AG1212-1AB23-2XB0: The 8 digital inputs are sink/source configurable per group, and the 6 outputs (2 of them pulse-train capable) switch in microseconds. 4 high-speed counters, each 30 kHz, 32-bit signed, can handle incremental encoders with two tracks offset by 90° – no extra counter module needed.
Backup: the integral capacitor holds program and retentive data about 50 hours (min 8 h at 40 °C). An optional battery module stretches that to 200 days – for long power-downs or transport. The EEPROM stores the whole program and DB1 permanently, so data doesn't vanish even if the capacitor drains completely. Input filter can be parameterized down to 0.2 ms for fast signals, or left at 12.8 ms for switch contacts. The four high-speed counters have their own dedicated interrupt subroutines – no ladder delay for encoder or pulse events.
If you're looking at 6AG1222-1BH32-2XB0 as a second source: that's a 16-point digital output module, not a CPU. They don't drop-in replace each other. The 6AG1212-1AB23-2XB0 is the PLC brain – the other is just output cards. Wrong swap would leave the panel without a controller.