Logic gates made from transistors diodes and resistors.
Discrete OR gate
There are three basic possibilities to build OR gate. Due to voltage decreases, such gates should not be connected serially (up to two can be used). The other disadvantage is influence from output to input - such feedback must be analysed.
1. The diode-diode-resistor OR gate (DDR). Resistor forces low state, if both A and B inputs are in low state (<0.8V). If any input is in high state (>2.7V), the output has high state. High-state voltage on the output is always 0.7V lower due to diode voltage decrease. High state apperars very fast, while return to low time depends on resistor and capacity connected to the output. More inputs can be added by connecting more dioded with cathodes joined together.
2. The resistor-diode OR gate (RD). Is simpler because requires less parts. The adantage is smaller voltage decrease on input with resistor. Also the return to low time is shorter (due to smaller resistor). More inputs can be added by connecting more dioded with cathodes joined together. Resistor can be only one.
3. The resistor-transistor OR gate (RT). The advantage is small current (high impedance) on transistor input and high output current (low impedance) if transistor input is in high state. The transistor can be driven with TTL HCT or HC output without problems. This solution is very popular in Sinclair computers (ROM disabling, ULA port decoding) and also CPC (ROM disabling). More inputs can be added by connecting more transistors with emitter joined together (collectors, too). Resistor can be only one.
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