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.
Two letter codes denote the CMOS equivalents of popular TTL devices. These are C (for CMOS) and HC (for high speed CMOS). While these will have the same pin numbers and functions as the true TTL devices of that part number, these are CMOS devices and must be used in the same way as other CMOS devices. The main advantage of "C" and "HC" devices is to let you use the equivalents to popular TTL devices in all-CMOS circuits. This is another good reason to start out with TTL; you can directly transfer your knowledge to CMOS devices. Some TTL devices will be described as open collector There are open collector equivalents to popular TTL devices; for example, the 7401 is identical to the 7400 except that the 7401 is an open collector device. The difference between the two is that the 7400 has output transistors integrated into it to provide a low output impedance regardless of whether the output is high or low, while the 7401 has no such transistors. When using open collector devices, an external resistor (known as a pull-up resistor) must be used between the output and any other device. The advantage of open collector TTL devices is that several different outputs can be "hard wired together to for a single output.

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.


7400 Quad two-input NAND Gate

7401

Open Collector Quad two-input NAND Gate
7402 Quad 2-Input NOR gate
Quad 2-Input NOR gate Quad 2-Input NOR gate
7405 Open Collector hex inverter
Quad 2-Input NOR gate Quad 2-Input NOR gate
7410 Triple 3-Input NAND Gate
7411 Triple three input AND gate
7413 Dual four-input Schmitt trigger
7414 Hex Schmitt trigger
7420 Dual four-input NAND gate
7421 Dual four-input AND gate
7427 Triple three-input NOR gate
7430 Eight- input NAND gate
Quad 2-Input NOR gate Quad 2-Input NOR gate
7437 Quad two-input NAND buffer
7441 BCD to decimal decoder
7447 BCD to seven segment decoder/driver
7448 BCD to seven segment decoder/driver
7451 Dual AND-OR invert gate
7453 Expandable AND OR invert gate
7454 Four-input AND OR invert gate
7473 Dual J-K flip-flop with clear
7474 Dual D-type positive edge triggered flip flop with reset and clear
7475 4-Bit Bistable Latch
7476 Dual J-K flip -flop with reset & clear
7485 Four-bit magnitude comparator
Quad 2-Input NOR gate Quad 2-Input NOR gate
7490 Decade counter
7491 Eight-bit shift register
7492 Divide by 12 counter
7493 Divide by 16 counter
7495 Four-bit left / right shift register
74109 Dual J-K positive edge triggered flip-flop
74121 Monostable multivibrator
74123 Dual retriggerable Monostable multivibrator with clear
74132 Quad 2-input NAND Schmitt trigger
74138 1 of 8 decoder multiplexer
74139 Dual 1 of 4 decoder
74151 I of 8 data selector
74153 Dual four-input multiplexer
74154 4 line to 1 line decoder

74157

Quad 2-input multiplexer
74161 Four bit up counter
74164 8 - Bit Shifter register
74165 Eight bit parallel to serial converter
74173 Four-bit D-type register
74174 Hex D-type flip-flop
74175 Quad D-type flip-flop
74190 Up/down decode counter
74191 Up/down binary counter
74192 Up/down decode counter with up down Clocks
74193 Up / down binary counter with up / down Clocks

74194

Four-bit shift register
74195 Four-bit shift register
74196 Presettable decode counter
74221 Dual Monostable multivibrator
74237 1 of 8 decoder / demultiplexer
74240 Octal buffer / line driver with three-state output
74241 Ocean buffer / line driver with three-state output
74242 Quad three state bus transceiver with inverting outputs
74243

Quad three-state bus transceiver with non inverting outputs

74244 Octal buffer / line driver with three-state output
74245 Ocean bus transceiver with three-state output
74251 Eight input data selector / demultiplexer with three state output
74253 Dual four input data selector / demltipexer with three state output
74257 Quad 2-Input data selector / multiplexer with three state output
74259 Eight-bit addressable latch / decoder
74273 Octal D-type flip flop with common clock and reset
74280 Nine-bit odd / even parity generator / checker
74283 Four-bit binary full adder
74299 Eight-bit bi-directional universal shift register
74356 Hex three-state buffer with common enable
74367 Hex three-state buffer
74368 Hex three-state bus driver
74373 Octal transparent latch with three-state output
74374 Octal D type flip-flop with three state output
74377 Octal D-type flip-flop with output enable
74386 Quad 2-input XOR gate
74393 Dual four stage binary ripple counter
74533 Octal three-state inverting transparent latch
74540 Octal three-state line driver / line receiver
74563 Octal three-state transparent latch

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

These Are The Truth Tables


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