Wired Logic in Digital Electronics
Wired logic is a type of digital logic where some logic operations are carried out by directly coupling the outputs of a single or several logic gates. This approach is determined by the electrical properties of the gates involved. A logic gate that uses just passive components like diodes and resistors to implement boolean algebra is known as a wired logic connection. An AND or an OR gate can be built using wired logic. Level restoration is absent, and it is impossible to build a NOT gate.
Wired Logic using a pull-up resistor and a diode
Wired-AND connection: The wired-AND connection is a type of AND gate. This function is built using a pull-up resistor and a diode for each input.
The diodes are used to conduct the positive voltage from the output C to the input sources A and/or B. The source voltage is directed to the output C when the positive voltage is greater than or equal to all of the input sources (i.e. A and B). This only applies to scenarios with two inputs because an AND gate may accept any number of inputs. According to the truth table, the output C is determined by inputs A and B.
Truth Table of AND gate:
Instead of diodes, open collector TTL gate outputs are used to implement the wired-AND operation. Simply connecting gate outputs to a common-collector pull-up resistor will enable the wired-AND operation.
Wired-OR connection: The wired-OR connection uses a pull-down resistor and one diode per input to achieve the Boolean logic action of an OR gate.
Voltage from any input is sent directly to output C through its corresponding diode. There will be no output if no voltage/logic is present on any input. In the OR connection, the positive voltage source present in the AND connection is replaced by a ground. Observe how the diodes are positioned concerning the AND gate as well. Similar to the AND connection, the OR connection only allows for one output and any number of inputs. According to the truth table, the output C is determined by the inputs at A and B.
Truth Table of OR gate:
Wired Logic using Open Collector gates:
An open-collector circuit is an electrical logic device implementation in which the device’s output is produced by the open-circuit collector termination of the output transistor. Because of this, the output of the device is active-low, and an active-high state needs to be established using a pull-up resistor. Using these components, wired-logic buses may be implemented or loads with high supply voltages can be driven.
Except for OR gates, open collector ICs are available for the majority of logic types, including AND, NAND, and so on. Open collector gates, on the other hand, can be utilized to provide wired AND and wired OR functions. If the outputs of two gates without open collectors are in opposing logic states, the logic 0 output of the gate will attempt to sink more current than the logic 1 gate can source, which will likely result in damage. As a result, the outputs of these gates should not be coupled together. When using open collector (or drain) gates, a gate output at logic 0 will be sinking current from the external pull-up resistor Re, and any other open collector gate that is connected and attempting to output a logic 1 will have its output transistor switched off and therefore not be sourcing any current.
Wired-AND function: If two or more open collector gate outputs are coupled together, any gate with a logic 0 output will pull all other connected outputs to logic 0, providing a logic 0 output at output X; however, if all connected outputs are at logic 1, then X will be at logic 1, the action of an ‘invisible’ AND gate.
Wired-OR function: It is also feasible to create a wired-OR function using open collector (or drain) gates, as illustrated in Fig wired-OR, though the reasoning is a little more complicated because Negative Logic is used. The circuit is utilized without employing a physical OR gate to obtain the Boolean function (A•B)+(C•D). Except for the two open collectors NAND gates in place of the two open collector AND gates, the circuit is quite similar to the wired-AND circuit. However, this circuit’s key distinction is the use of Negative Logic to produce an OR function from what initially looks to be an AND function.
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