Logic chips are semiconductor integrated circuits that can provide logic functions AND, OR, and XOR and their opposite counterparts NAND and NOR. They have been popular since the ’60s and were best known as the 74 series family (and 54 series in military-grade).
They have migrated through several families, such as the ECL, TTL, CMOS and combinations. In addition to the basic logic gate functions, there are also adders, flip-flops, Schmidt triggers, and other functions.
The ’60s saw logic chips appear in early minicomputers such as the PDP and most of the space flights from Gemini to the Shuttle.
Shown below are some logic boards used in the space program.
74 Series Families
The early 7400 TTL (transistor-to-transistor logic) series evolved to 74LS (low power Schottky). These were 5V-only devices and were followed by 74HC, combining the speed of TTL with the low power consumption of the 4000 series CMOS and retaining pin compatibility with TTL footprint. However, interfacing between families has to be considered with care (more to follow on this).
After 74HC, we have 74HCT which overcame some of the interfacing problems and can be mixed with 74LS. The 5400 series are military-grade versions of the 74 series. The 4000 series can work over a very wide range of input voltages (up to 18V).
Logic Symbols Truth Tables and Functions
Logic signals are often expressed as 1’s and 0’s, High and Low, 0V and 5V, True and False, and even On and Off. These all refer to the same logic levels.
Shown below are some logic gates, and the input levels and resulting output levels.
Interfacing Logic Chips
It is good practice and sensible to ground all unused inputs, especially with CMOS. Also, make sure to take care in handling CMOS in dry or electrostatic possible situations.
When building up a digital system, and particularly when breadboarding and relying on chips you have lying around, the temptation often arises to “mix and match” families, like using 74LS together with 74HC, etc. But there is a special case that has to be understood as the TTL output levels may not always be compatible with CMOS input level requirements.
See below (images used with permission from allaboutcircuits.com):
In the example above at the left, the TTL output level on a low falls below the CMOS input requirement and will work properly. But on the right, the TTL level on a high output may not always fall within the CMOS input requirement and will not be interpreted as a high. The solution shown below is to insert a resistor “pullup” at the input of the CMOS gate to force it into the acceptable high region. A value of 10k will do. Using 74HCT also solves this problem.
Some Common Pinouts for Logic Chips
We have looked at various families and types of logic gates. A lot of contemporary design is being taken away from gates with the advent of cheap and easy-to-program micros such as an Arduino. However, many functions are still better off using discrete gates. Gates are very cheap and versatile. By biasing the input of a gate, it can even work as an analog amplifier! CMOS is not limited to 5V and can be used when only a 9V battery is available.