Tri-state logic: Connecting multiple outputs together - 8 bit register - Part 2

Introduction to Bus Outputs

In this section, the importance of ensuring only one module is outputting to the bus at a time is discussed. The potential problems that can arise when multiple modules output simultaneously are highlighted.

Understanding Bus Outputs

• It is crucial to have only one module connected to the bus outputting at any given time.

• If multiple modules output simultaneously, conflicts and ambiguity in input values can occur.

Examining Logic Gate Output Stage

• Logic gates form the basis of bus connections.

• Inside a logic gate, there are transistors that perform specific logic operations.

• An output stage exists in front of the actual gate output.

• The output stage typically consists of two transistors that control current flow.

Sourcing and Sinking Current

• When the top transistor is turned on, current flows from the voltage source through to the output. This represents sourcing current (output = 1).

• When the bottom transistor is turned on, current can be sunk into the ground. This represents sinking current (output = 0).

Current Flow and Input Determination

• Current flowing in one direction indicates an input value of 1.

• Current flowing in the opposite direction indicates an input value of 0.

Conflicts with Multiple Outputs

• When multiple outputs try to drive conflicting values simultaneously, issues arise.

• For example, if one module outputs a 1 while another tries to sink current (output = 0), conflicts occur.

Disabling Outputs with Tri-State Gates

• Tri-state gates provide three output states: 0, 1, and disabled (disconnected).

• By not turning on either transistor in a tri-state gate, the output remains disconnected.

Explanation of Tri-State Gates

This section explains the concept of tri-state gates and their role in disconnecting outputs when not enabled.

Tri-State Gates

• Tri-state gates have three output states: 0, 1, and disabled (disconnected).

• They are commonly represented by a symbol with an input, output, and enable pin.

• The enable pin controls whether the output is connected or disconnected.

Example: 74LS245

• The 74LS245 is an example of a chip that contains multiple tri-state gates.

• It allows for enabling or disabling specific outputs as needed.

Conclusion

In this final section, the importance of using tri-state gates to disconnect outputs when not in use is emphasized.

Importance of Tri-State Gates

• Tri-state gates play a crucial role in preventing conflicts and ensuring proper bus operation.

• By disconnecting outputs when not enabled, they prevent unwanted current flow and ambiguity in input values.

Logic and Enable Lines

This section explains the concept of a bus and how enable lines are used to control data flow between modules connected to the bus.

Bus and Enable Line

• A bus allows multiple modules to communicate with each other.

• Each module is connected to the bus through an enable line.

• By setting the enable line low for all but one module, conflicts on the bus can be avoided.

• Only the enabled module will output data to the bus.

• When another module reads data from the bus, it will receive data from the enabled module.

Flexibility of Bus Connections

This section emphasizes the flexibility provided by connecting modules to a bus and introduces registers as a way to store and retrieve data from the bus.

Benefits of Bus Connections

• Modules connected to a bus can communicate with any other module on the same bus.

• Adding multiple modules becomes easier due to this flexibility.

Introduction to Registers

• Registers are used to store and retrieve data from a bus.

• Data can come from or go to the bus, controlled by control lines.

• The source of these control lines is not discussed yet.

Enabling Output and Loading Data

This section explains how enabling output and loading data work in relation to registers connected to a bus.

Enabling Output

• An enable control line determines when stored data should be outputted onto the bus.

• Only one enable line should be high at any given time.

Loading Data

• A load signal indicates that on the next clock pulse, new data should be read into the register.