Logic level reference
Interactive engineering and procurement calculator. Instant client-side computation with no sign-up.
Logic family input/output thresholds — typical datasheet values
| Family | VOH | VIH | VIL | VOL |
|---|---|---|---|---|
| TTL (5 V) | 2.4 V | 2 V | 0.8 V | 0.4 V |
| 74HCT (5 V) | 4.9 V | 2 V | 0.8 V | 0.1 V |
| 5 V CMOS (74HC) (5 V) | 4.9 V | 3.5 V0.7·VCC | 1.5 V0.3·VCC | 0.1 V |
| LVTTL (3.3 V) | 2.4 V | 2 V | 0.8 V | 0.4 V |
| LVCMOS 3.3 V (3.3 V) | 3.1 V | 2 V | 0.8 V | 0.2 V |
| LVCMOS 2.5 V (2.5 V) | 2.3 V | 1.7 V | 0.7 V | 0.2 V |
| LVCMOS 1.8 V (1.8 V) | 1.35 V | 1.17 V0.65·VCC | 0.63 V0.35·VCC | 0.45 V |
| RS-485 | differential — A−B high ≥ +1.5 V typ (min +0.2 V); receiver threshold +0.2 / −0.2 V; low ≤ −1.5 V typ | |||
| LVDS | differential — Driver VOD 0.25–0.45 V (typ 0.35) — worst-case 0.25 / −0.25; receiver threshold ±0.1 V; common mode ≈ 1.2 V | |||
Typical JEDEC-style values for reference. Actual thresholds vary by part, temperature and supply — always check the specific datasheet. The differential families are quoted as the A−B voltage (RS-485) or the differential output voltage VOD (LVDS).
Driver → receiver compatibility check
Verdict
VOH ≥ VIH and VOL ≤ VIL
High margin (VOH − VIH)
2.4 V − 3.5 V
Low margin (VIL − VOL)
1.5 V − 0.4 V
Not compatible — VOH 2.4 V is below the receiver VIH of 3.5 V (high state fails). Use a level shifter.
Noise margin
High-state noise margin
Family self-margin: VOH − VIH
Low-state noise margin
Family self-margin: VIL − VOL
The self-margin shows how far a logic output can be pushed before the same family's own input stops reading it. The pair check above compares two different families, which is the margin that matters when mixing TTL, CMOS and LVCMOS on one bus.
About logic levels
A logic family is only as interoperable as its thresholds: the driver must push above the receiver's VIH for a reliable high and pull below its VIL for a reliable low. Classic TTL, with a 2.4 V VOH, cannot drive the 3.5 V VIH of 5 V CMOS directly — the classic reason level shifter ICs exist. Keeping one family for an entire rail, or checking the margin like above, avoids the intermittent glitches that marginal interfaces produce.
For the parts themselves: level shifters and logic gate ICs carry the exact thresholds this check assumes.
Browse related parts: