For a TTL-type input, a logical zero is between ground and 0.8V; a logical one is between 2.2V and VCC (on some devices, the thresholds may be slightly lower for lower voltage ranges). CMOS input switching thresholds are usually specified proportionally to VCC (for instance, zero could be anything less than 0.35*VCC and one could be anything greater than 0.65*VCC). These thresholds are typically specified in the electrical specifications, but may not be called out as CMOS. Look for "input voltage, high" and "input voltage, low".
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