Logic Families and Interfacing
Two logic ICs can perform the same truth table and still be electrically incompatible. The family name tells you much about supply voltage, thresholds, speed, current, and power.
| Family | Construction | Typical role |
|---|---|---|
| 74LS | Bipolar Schottky TTL | Classic 5 V systems. |
| 74HC | High-speed CMOS | General CMOS logic; CMOS thresholds. |
| 74HCT | High-speed CMOS | CMOS with TTL-compatible inputs at 5 V. |
| CD4000 | CMOS | Wide-supply, moderate-speed general logic. |
| 74AC / ACT | Advanced CMOS | Faster 5 V-class logic; ACT uses TTL-compatible thresholds. |
| 74LVC | Low-voltage CMOS | Fast 1.65–3.6 V-class systems; many devices tolerate higher input voltage. |
VOH and VOL
Datasheets guarantee an output HIGH above a minimum voltage VOH and an output LOW below a maximum voltage VOL under stated load currents.
VIH and VIL
An input is guaranteed HIGH above VIH and guaranteed LOW below VIL. The region between them is undefined.
Noise margin
LOW noise margin = VIL(max) - VOL(max)
A positive margin means some noise can be added without changing the interpreted logic state.
TTL output into CMOS input
Classic TTL HIGH voltage may not reach the guaranteed HIGH threshold of a 5 V 74HC input. This is one reason 74HCT exists: its CMOS circuitry accepts TTL-like input thresholds.
3.3 V and 5 V systems
A 3.3 V output may or may not be recognized as HIGH by a 5 V input, and a 5 V output may damage a 3.3 V input. Compatibility must be checked in both voltage directions and at power-up.
Level translators
Dedicated translators handle mismatched logic supplies. Some are direction-controlled; others automatically detect signal direction. Open-drain buses such as I²C can often be translated with specialized MOSFET-based circuits.
Open collector and open drain
These outputs actively pull only one direction and use a resistor or current source for the other. They are excellent for wired sharing, interrupt lines, reset networks, and level translation.
Three-state outputs
A three-state output can drive HIGH, drive LOW, or electrically disconnect into a high-impedance state. This allows several devices to share a bus as long as only the intended device drives it at a time.