Logic Families – Notes – 1

1. Digital or logic circuits operate on voltages that fall in prescribed ranges that represent either a binary 0 or a binary 1.

2. All digital logic devices are similar in nature but very much different regarding the details of their characteristics. An understanding of the terms used to describe these characteristics is important and allows us to compare and contrast the performance of devices. By understanding the capabilities and limitations of each type of device, we can intelligently combine devices to take advantage of each device’s strengths in building reliable digital systems.

3. The TTL family of logic devices has been in use for over 30 years.The circuitry uses bipolar transistors. This family offers many SSI logic gates, and MSI devices. Numerous series of similarly numbered devices have been developed because advances in technology have offered improved characteristics.

4. When you are connecting devices together, it is vital to know how many inputs a given output can drive without compromising reliability. This is referred to as fan-out.

5. Open-collector and open-drain outputs can be wired together to implement a wired-AND function. Tristate outputs can be wired together to allow numerous devices to share a common data path known as a bus.In such a case, only one device is allowed to assert a logic level on the bus (i.e., drive the bus) at any one time.

6. The fastest logic devices are from a family that uses emitter-coupled logic (ECL). This technology also uses bipolar transistors but is not as widely used as TTL due to inconvenient input/output characteristics.

7. MOSFET transistors can also be used to implement logic functions. The main advantage of MOS logic is lower power and greater packing density.

8. The use of complementary MOSFETs has produced CMOS logic families. CMOS technology has captured the market due to its very low power and competitive speed.

9. The ongoing need to reduce power and size has led to several new series of devices that operate on 3.3 V and 2.5 V.

10. Logic devices that use various technologies cannot always be directly connected together and operate reliably. The voltage and current characteristics of inputs and outputs must be considered and precautions taken to ensure proper operation.

11. CMOS technology allows a digital system to control analog switches called transmission gates.These devices can pass or block an analog signal, depending on the digital logic level that controls it.

12. Analog voltage comparators offer another bridge between analog signals and digital systems.These devices compare analog voltages and output a digital logic level based on which voltage is greater.They allow an analog system to control a digital system.

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