Computer Science Principles V11

Clive W. Humphris

LOGIC FAMILIES: DL (Diode Logic).  

Digital circuits are normally found using integrated circuits as the building blocks. However simple logic circuits can be constructed from discreet components. In fact it can help to further your understanding of logic as the voltages and currents can be measured directly. Please note that throughout the Logic Family topics topics inputs A and B can apply to either terminal.

Observe the equivalent AND gate on the left first. Logic LOW on either cathode forward biases that diode which in turn pulls the output voltage LOW via R1, likewise logic HIGH means the diode is cut OFF. Whilst this will satisfy an equivalent AND gate condition where both diodes must be biased OFF, the output logic levels are dependant on the value of R1 and the resistor load. Also zero volts for logic 0 can never be achieved. Nevertheless the circuit could still be used where an IC version is impractical.

The OR gate takes a supply current from either or both diodes to generate a voltage across R2. Note the there can be any number of diode inputs for these AND/OR circuits.

Whilst the switching changeover (propagation delay) rate on any IC can never be instantaneous, these simple circuits have additional limitations if speed or accuracy is important. The effects of stray circuit capacitance will upset the rectangular waveform shape, which restricts Diode Logic to low speed applications. But not all logic has to be fast. Take a burglar alarm for example a few seconds delay when closing a door makes little or no difference.

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