I have followed several posts that suggest using a single resistor, voltage dividers, zener diodes, and opto-isolation. Can anyone direct me to a simple-but-adequate method to protect the digital-input while still getting reliable serial data?
Most PC COM ports put out +/-12vDC, but the industrial PC I am using for this Nano project puts out 6.0vDC. Close to the 5.5v max, but still too much for a direct-connection.
Thank you, and forgive me if I just can't find the answer in the Forum.
Buy an RS232 converter. Preferably one with a Maxim chip in it. Often the MAX232 is referred to, even though the actual chip is a different one from the MAX232 family.
The reason nothing you have tried works is because the RS232 signal you have is upside down to the TTL signal you need. When one is high the other is low, and with RS232 low means a negative voltage which will damage your Arduino.
Standard series-resistor plus shottky rail diodes should work nicely - its general purpose, although larger
values of resistor like 10k limit the bandwidth (not a problem with serial, normally).
SoftwareSerial does have an option for "inverted serial" which will allow you to read the data coming from the simple resistor/diode type converter. It can't transmit, unless the receiver is extremely lenient in its interpretation of the standards.