TTL Devices

Transistor-transistor logic (TTL) IC's were the first digital logic ICs to achieve widespread use. Earlier device "families," such as the resistor- transistor logic (RTL) and diode-transistor logic (DTL), pioneered the use of logic ICs but had too many shortcomings. TTL was faster, cheaper, and more reliable than RTL and DTL. TTL technology was developed by Texas instruments and by the mid 1970's the dominant digital logic technology. Today, TTL has been largely supplanted by CMOS for most applications. However, it is still a good choice for experimenters and as an inexpensive way to master digital electronics. Compared to CMOS, TTL devices are relatively inexpensive and have good immunity to damage due to static electricity discharges. Other features of TTL include fair noise immunity, good speed, and good "fan-in" (number of input signals an input can accept) and "fan-out" (number of output signals an output can provide) capabilities. While CMOS is the preferred logic family for most applications, TTL will likely remain in use for many more years. The major disadvantage of TTL is its heavy current consumption. Although versions of TTL consuming less current have been developed, all are still inferior to CMOS. The heavy current demands of TTL can introduce "glitches" and other variations into the power supply when a TTL device changes logic states and draws a heavy load of current. This can result in false triggering and switching of TTL gates. Component layout and interconnection is critical in high speed circuits using TTL, and decoupling capacitors are frequently necessary. TTL also requires a constant +5 volt power supply, known as Vcc, and a TTL IC will quickly be destroyed if this is exceeded.

TTL devices can be easily identified by their part numbers. These normally consist of four or five digits and begin with the number "7." (Some early TTL devices had part numbers beginning with "5," and these may still be encountered.) A handful of TTL devices had part numbers beginning with "8," but most of these also have equivalent part numbers beginning with "7." Letter codes are often inserted into TTL part numbers after the first two digits to indicate which sub-family of TTL devices a part belongs to, although in some cases the device is not really TTL! The various sub-families have the same pin connections and functions as ordinary TTL devices, although they are operationally different and generally cannot be used in the same fashion as ordinary TTL. Here is a list of these letter codes and their meaning:

• LS. This denotes a low power Schottky TTL device, a popular subfamily that uses only about 20% of the current of standard TTL and also operates faster. However, it is more expensive than standard TTL. For many applications, LS TTL is a good compromise between performance and cost. LS TTL is probably the most widely used version of TTL in the world today, and is highly recommended, especially for circuits powered from batteries.
• ALS. This stands for advanced low power Schottky, an improved version of LS TTL that is significantly faster than LS and lowers the power consumption level to a point competitive with CMOS. This subfamily is very popular with professional engineers working on advanced commercial and military Projects, but is too expensive for most hobby and experimenter purposes. If lower power consumption is important, CMOS or LS TTL is usually a better choice.
• F, This is a fast TTL IC, and was an early attempt to improve TTL's Speed. It is seldom found today.
• H, This stands for high speed, an early TTL subfamily with improved speed but higher power consumption. It is seldom found today.
• S, This stands for Schottky, an early version of LS TTL that's rare today.

The Table below lists some of the more common TTL devices by part number. The sheer number of TTL devices means we can only display a certain number of IC's.

7401

74157

74194

Quad three-state bus transceiver with non inverting outputs

The supply voltage (Vcc) requirements for TTL are critical. Vcc must be held as close to +5 volts as possible; if it exceeds +5.25 volts, the TTL device will likely be destroyed A TTL IC may refuse to function if Vcc drops below +4.75 volts. Good supply voltage regulation is essential in all circuits using TTL devices, To prevent voltage spikes from damaging TTL ICs, connect a capacitor between l to 10 µF across the power supply leads where they make contact with a solderless breadboard.

The heavy current demands of TTL can cause improper operation due to power supply variations as the devices switch output states. To prevent this, decoupling capacitors should be placed across the Vcc and ground pins of each TTL device when a circuit uses more than two TTL devices. 0.1 µF is the best value for decoupling capacitors, and their leads should be kept as short as possible. The need for decoupling capacitors increases when higher speed versions of TTL like LS and ALS are used.

Most TTL devices will treat any input signal in excess of +2 volts as a high level logic signal while any input below +0.8 volts will be consider as a low level logic signal. For the most reliable operation, let Vcc (+5 volts) represent a high level logic with ground (0 volts) representing a low logic level. All inputs must not exceed Vcc or the TTL IC will be destroyed.

All unused inputs on a TTL IC should be connected to Vcc to prevent erratic operation of the IC. If battery power supply is used or current consumption is a critical factor, the outputs of any unused logic gates on the TTL device should be set to high by connecting the inputs of the gates to Vcc or ground as necessary to produce a high output.

It is possible to apply the same input signal to two or more inputs by connecting all inputs to a common point to which the input signal is applied. The "fan out" capabilities vary with the version of TTL used. As good working practice, assume that a conventional TTL output can drive no more than ten conventional TTL or twenty LS TTL inputs. A LS TTL output can drive half the number of inputs (five conventional TTL or ten LS inputs). If a device is operated at these fan out limits and the circuit is not functioning properly, try reducing the number of inputs driven by the output. There are several terms used to describe the performance of TTL devices. One is propagation delay, which refers to how quickly a change in a device's inputs will produce a change in the device's output level. Propagation delay is sometimes referred to as response time. Toggle frequency is how fast the output(s) of TTL device can switch between high and low logic states.

Below is a table of the Logic Gates