TTL, which means transistor transistor logic, is most commonly used for small and medium-scale integration (MSI). There are two basic forms of TIL. The first and original group has a low-power/low-speed version and the standard line. The later designs are Schottky clamped, which come in quite a few versions and are still growing. The Schottky diodes are used to prevent the transistors from going into saturation, with a resulting increase in speed for a given power dissipation.
The older families, including the 54H/74H are obsolete and used only for replacement. The low-power Schottky, 54LS/74L5, and the standard Schottky, 54S/74S, have been around for a long time. The newer versions include the family 54F/74F called FAST and two Texas Instruments versions, 54AS/74AS and 54ALS/74ALS. There are designated Advanced Schottky and Advanced Low Power Schottky, respectively. All of the TIL families represent different tradeoffs between power consumption and speed.
If speed is not required, the Low Power class is a good choice, not only because of low power consumption, but because its low speed makes it insensitive to many high-frequency spikes and glitches.
Although open inputs to TTL logic act as a high, it is not a good practice to leave them disconnected in final circuitry. This is because the maximum speed is lowered and the noise susceptibility is increased. It is also not a safe practice to tie these inputs to the 5 V line because the breakdown voltage of the input lines are only 5.5V compared to 7 V for the supply pin. Unused open inputs can be connected to the output of a spare gate that is held at a high or a 1 kH resistor can be inserted between the gate lead and the + 5 V supply. One resistor can be used for up to 25 gates in some logic families. A better way, however, is to use two resistors and a couple of microfarads of capacitance to form a stable Hi of about 3.5 V.
One exception to this is the LS series where most of the devices have diode inputs, which can be directly connected to the + 5 V supply. Some of them have emitter inputs, however, which must be connected through the 1 kH resistor.
The families are almost completely pin compatible, but for some strange reason there are a few exceptions. The power pin for dual-in-line sockets is usually # 16 (or # 14) and the ground pin # 8 (or # 7) depending on whether it is a 16-pin or 14-pin package. But, again, not always.
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