by 1973 there was no mention of the 6400 family in the Texas Instruments TTL Data Book. Some companies have also offered industrial extended temperature range variants using the regular 7400-series part numbers with a prefix or suffix to indicate the temperature grade. As integrated circuits in the 7400 series were made in different technologies, usually compatibility was retained with the original TTL logic levels and power supply voltages. An integrated circuit made in CMOS is not a TTL chip, since it uses field-effect transistors (FETs) and not bipolar junction transistors, but similar part numbers are retained to identify similar logic functions and electrical (power and I/O voltage) compatibility in the different subfamilies. Over 40 different logic subfamilies use this standardized part number scheme. The headings in the following table are: Vcc – power supply voltage; tpd – maximum gate delay; IOL – maximum output current at low level; IOH – maximum output current at high level. tpd, IOL, and IOH apply to most gates in a given family. Driver or buffer gates have higher output currents. Many parts in the CMOS HC, AC, and FC families are also offered in "T" versions (HCT, ACT, and FCT) which have input thresholds that are compatible with both TTL and 3.3 V CMOS signals. The non-T parts have conventional CMOS input thresholds, which are more restrictive than TTL thresholds. Typically, CMOS input thresholds require high-level signals to be at least 70% of Vcc and low-level signals to be at most 30% of Vcc. (TTL has the input high level above 2.0 V and the input low level below 0.8 V, so a TTL high-level signal could be in the forbidden middle range for 5 V CMOS.) The 74H family is the same basic design as the 7400 family with resistor values reduced. This reduced the typical propagation delay from 9 ns to 6 ns but increased the power consumption. The 74H family provided a number of unique devices for CPU designs in the 1970s. Many designers of military and aerospace equipment used this family over a long period and as they need exact replacements, this family is still produced by Lansdale Semiconductor. The 74S family, using Schottky circuitry, uses more power than the 74, but is faster. The 74LS family of ICs is a lower-power version of the 74S family, with slightly higher speed but lower power dissipation than the original 74