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GROUP 6

MSI LOGIC CIRCUITS

        -Magnitude Comparator

        -Code Converters

        -Data Busing

-Potential Exam questions and answers.

Whenever you see something like OE(bar) I mean

MAGNITUDE COMPARATORS

The magnitude comparator is a combinational logic circuit that compares two input binary quantities and generates outputs to indicate which has the greater magnitude.

please try to understand this truth table.

CASCADING INPUTS

Cascading inputs provide a means for expanding the comparison operation to more than 4-bits by cascading 2 or more 4-bits comparators.

APPLICATIONS

Magnitude comparators are useful in control apps where a binary number representing the physical variable being controlled(speed or temperature) is compared with a reference value.

• Check out the DIGITAL THERMOSTAT example

CODE CONVERTERS

A code converter is a logic circuit that changes data presented in one type of binary code to another type of binary code. The BCD-to-7 segment decoder-driver is a code converter because it changes a BCD input code to the 7-segment code needed by the LED display.

READ BASIC IDEA and CONVERSION PROCESS (624 and 625)

DATA BUSING

In modern computers, the transfer of data takes place over a common set of counting lines called DATA BUSES.

Different devices can have their output and inputs tied to the common data bus line.

The data bus is simply a collection of conducting paths over which digital data are transmitted from one device to another.

Each device provides an 8-bit output that is sent to the inputs of the microprocessor over the 8-line data bus.

THE 74ALS173/HC173 TRISTATE REGISTER

The devices connected to a data bus will contain registers (usually flip-flops)that hold the device data. The outputs of these registers are usually connected to tristate buffers that allow them to be tied to a data bus. We will demonstrate the details of data bus operation by using an IC register that includes the tristate buffers on the same chip: the TTL 74ALS173 (also available in CMOS 74HC173 versions).

DATA BUS OPERATION

NOTE:

• OE一 and IE inputs (pair) are tied together as one input
• The registers will be referred to as A,B, and C from top to bottom.

In this arrangement, the data bus consist of 4lines labeled DB1-DB3.

Corresponding outputs of each register are connected to the same bus line (03A, 03B, 03C are connected to DB3) likewise 02A, 02B and 02C …

Because the 3 registers have their outputs connected together, It’s very important that only one register has its output enabled and the 2 other register outputs remain in Hi-2 state, Otherwise, there would be BUS CONTENTION. Bus Contention is where two sets of outputs fight each other and as a result, produce uncertain levels on the bus and possible damage to the register output buffers.

Also, DBA, DBB, DBC (register inputs) are tied to the same bus lines. Thus, the levels on the bus will always be ready to be transferred to one or more of the registers depending on the IE(bar) inputs.

DATA TRANSFER OPERATION

The contents of any one of the 3registers can be parallel-transferred over the data bus to one of the other registers through proper apps of logic levels to the register enable inputs.

In a computer, the CPU will generate signals that select which register will put its data on the data bus and which one will take data from the data bus.

READ THE FOLLOWING SUBTOPICS

• Bus Signals
• Simplified Bus Timing Diagram
• Expanding the Bus
• Simplified Bus Representation
• Biderectional Busing

POTENTIAL EXAM QUESTIONS

1 or 2 of any of these questions MIGHT be potential exam questions.

• What is the purpose of cascading inputs of the 74HC85          

Answer

To provide a means for expanding the compare operations to numbers with more than four bits.

• What are the outputs of a 74HC85 with the following inputs A3A2A1A0 = B3B2B1B0 = 1001, IA>B = IA<B = 0 and IA=B = 1?

        Answer

        OA=B = 1; other outputs are 0.

• What is a code converter?

Answer

A code converter takes input data represented in one type of binary code and converts it to another type of binary code.

• How many binary outputs would a three-digit BCD-to-binary converter have?

Answer

Three digits can represent decimal values up to 999. To represent 999 in straight binary requires 10 bits.

• What is meant by the term data bus?

        Answer

        A set of connecting lines to which the inputs and outputs of many different devices can be connected.

• What is bus contention, and what must be done to prevent it?

Answer

Bus contention occurs when the outputs of more than one device connected to a bus are enabled at the same time.

It is prevented by controlling the device enable inputs so that this cannot happen.

• What is a floating bus?

Answer

A condition in which all devices connected to a bus are in the Hi-Z state.

• What will happen if OEA(bar) = OEB(bar) = LOW in the figure above?

Answer

Bus Contention.

• What logic level is on a data bus line when all devices tied to the bus are disabled?

Answer

Floating, Hi-Z

• What is the function of a bus driver?

Answer

Provides tri-state low impedance outputs

• What are the reasons for having registers with common I/O lines?

Answer

Reduces the number of IC pins and the number of connections to the data bus.

• Redraw the figure above using bundled line representation.

Answer

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