According to T22H-npn installation guide:
For normal operations the T22H-npn controller requires a single +24V (? 15%) DC regulated supply connected to the upper-right screw terminal block as shown in Figure 1. Although the PLC may still operate at power supply voltage as low as 12V DC, the power-failure retentive relay feature (enabled when DIP switch #1 is ON) will not work for power supply voltage below 18V.
So if you don't require the retentive relay feature (such that when the power failure occur, some internal relays, timers and counters are retentive) the T22H-npn could still work at 12V DC, although it is not a recommended operating voltage. In general, the power supply should not be allowed to drop below 10.5V to prevent potential false triggering of the NPN inputs.
