Two Resistor Serial Interface - rosco-pc/propeller-wiki GitHub Wiki

Just as it is possible to "Interface 5V signals to the Propeller" higher than 5V signals can be interfaced, providing current limiting is enforced. This can be done using a single resistor.

This can be used to provide a minimal component count, low-cost serial interface for PC or other device connection ( for run-time communications, not for program downloading ).

A +/-12V RS232 receive line can be connected through just a single resistor and a Propeller output pin can usually drive an RS232 transmit line. This is the absolute minimum circuit and is not recommended. The receive line should be pulled down to prevent the input pin from floating when the serial cable is disconnected, negative input voltages can be blocked by a simple diode, a diode clamp will keep the input pin within accepted voltage range and help protect the internal clamping diodes from any adverse effects, the output pin can be protected by a current limiting resistor.

The author has used the following circuit with no apparent adverse effects for an extended period of time -

                                     ____
                    .---------------|____|-------------< Pout
                    |                330R
                    |                             1N4148
           .---.    |                        .---|>|---> 3V3
TX to PC   | O |<---'                ____    |
RX from PC | O |--------|>|-----.---|____|---^---------> Pin
0V         | O |----.  1N4148   |    560K
           `---'    |          .|.
                    |          | | 10K
                    |          |_|
                0V _|_         _|_

The Propeller Chip is conservatively rated to withstand +/-500uA injection current and the 560K resistor ensures that the injection current is considerably below that ( +/-21uA at +/-12V ).

The circuit can be reduced to just the 560K and 10K, hence the description as a "two resistor interface".

The interface is not RS232 specification compliant and may not be suitable for all circumstances. In particular, the output from the Propeller Chip will be 0V/+3V3 which may be too low for some receiver devices, however this has not been found to be the case with serial interfaces the author has tested.

Link : "Original thread"