74HCT series is a high-performance CMOS logic series, compatible with the pin and function of 74 series HC logic, but the input level is compatible with TTL
This document introduces the 54HC/74HC series of high-speed CMOS logic and its sub-family, 54HCT/74HCT. It details the specific design of the HCT sub-family to provide TTL-compatible input levels, addressing the incompatibility between standard HC inputs and TTL output high levels. The text explains that while standard HC logic drives TTL easily, it cannot reliably accept high-level signals from TTL outputs without additional circuitry like pull-up resistors.
| Output High Voltage (HC/HCT) | 3.7V |
|---|---|
| Output Low Voltage (HC/HCT) | 0.4V |
| Nominal Trip Point Voltage | 1.4V |
HCT is needed because standard HC inputs require a minimum of 3.15V for a logic high, which is not guaranteed by TTL outputs that only reach 2.4V.
Yes, HCT can be used as a direct plug-in replacement for 54LS/74LS logic, often reducing power consumption by a factor of 5.
The main tradeoff is a degraded ground level noise margin compared to standard HC, as the trip point is set to 1.4V to match TTL levels.