Logic Level Thresholds

Input thresholds (VIH/VIL) and guaranteed output levels (VOH/VOL), in volts. A driver's VOH must exceed the receiver's VIH with margin — this is why a 1.8 V part cannot reliably drive a 5 V CMOS input, and why 5 V outputs need level shifting into 3.3 V parts.

Family	VDD	VIH min	VIL max	VOH min	VOL max
TTL (5 V)	5.0	2.0	0.8	2.4	0.4
LVTTL (3.3 V)	3.3	2.0	0.8	2.4	0.4
CMOS (5 V)	5.0	3.5	1.5	4.4	0.5
LVCMOS (3.3 V)	3.3	2.0	0.8	2.9	0.4
LVCMOS (2.5 V)	2.5	1.7	0.7	2.0	0.4
LVCMOS (1.8 V)	1.8	1.17	0.63	1.35	0.45

Differential / serial standards

Standard	Signalling	Typical use
RS-232	±3 … ±15 V, single-ended	Legacy point-to-point, short runs
RS-485	Differential, 200 mV threshold	Multi-drop industrial, long runs, 120 Ω terminated
CAN	Differential, dominant/recessive	Vehicle and machine networks, 120 Ω each end
LVDS	Differential, ~350 mV swing	High-speed, low-noise board/cable links

5 V-tolerant inputs are a per-pin property — check the datasheet, never assume.

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