I know this may be a dumb question but can you connect a RS232 Tx, Rx, and Ground from a device directly to the Rx, Tx and Ground on a Raspberry Pi Zero without having to get a Max3232 or something similar to that?
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Given the significant differences in the voltage levels it is not safe to directly connect any RS232 signal lines to the Pi's GPIO. Even if the voltage of some RS232 drivers might be as low as 3 V, the logic one (called "mark") is represented as a negative voltage which will kill the Pi.
RS232 (Source):
The standard specifies a maximum open-circuit voltage of 25 volts: signal levels of ±5 V, ±10 V, ±12 V, and ±15 V are all commonly seen depending on the voltages available to the line driver circuit. Some RS-232 driver chips have inbuilt circuitry to produce the required voltages from a 3 or 5 volt supply. RS-232 drivers and receivers must be able to withstand indefinite short circuit to ground or to any voltage level up to ±25 volts. The slew rate, or how fast the signal changes between levels, is also controlled.
Raspberry Pi GPIO:
GPIO voltage levels are 3.3V and are not 5V tolerant. Voltages below 0V and above 3.3V are outside the operating voltage range.
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Ghanima
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In addition to the voltage difference, even if you limited it to 0 - 3V it would not work as the polarity is inverted – Milliways Mar 05 '19 at 21:14