Here is a quick summary of various means for serial communications and the resulting pin counts.
Different variations of serial communications connectors.
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SPI is the simplest type because it can connect straight to a shift register. How many pins? Typically 7 to 8: Ground, 3.3 V, 5 V (optional), RESET, Slave Select, CLOCK, MOSI, MISO.
If we omit RESET and use power-cycling for RESET instead, assuming the receiver has a power-on-reset circuit, then we can go with only 6 pins.
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Asynchronous serial communications. Eliminating the separate clock signal allows for only 5 pins: Ground, Power, Data Terminal Ready (DTE), Transmit, Receive.
To get to 4 pins, DTE can be eliminated by assuming a bus protocol that transmits that information “in-band,” i.e. software flow control rather than hardware flow control.
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Asynchronous serial communications, half-duplex. 3-state logic and state tracking is used to perform transmit and receive on the same conductor. 3 pins.
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Multiplexed power and data transmission, “powerline communications.” Data transmission is modulated on the power line. 2 pins.
- Single-wire earth return, half-duplex powerline communications. This is technically not feasible in portable hand-held electronics since they are low-voltage and may have no earth return path, especially when they are airbourne.