I have a similar question, so I am going to post it into this thread.
One UART of my Pi Zero (or Raspi3B, no matter) is already occupied by a power metering device.
Now I want to install an additional communication with a peripheral (Arduino Nano, or ESP8266), which has to be isolated by an optocoupler.
How can this be achieved most easily?
I guess, Rx/Tx is the signal that can be passed through an optocoupler most easily; low Baud rate is enough. But then, can I activate a second serial port in the Raspi, perhaps over USB, in addition to the usual UART pins?
Alternatively, SPI or I2C is considered, but SPI requires more wires to be coupled, and I2C deserves a more expensive optical coupler.
In short: Can I use more than one (asynchronous) UART (Rx/Tx) port in the same time on the pi?
edit: The choice of using a Pi Zero or a Raspi 3B does matter. If using a Pi Zero, the only USB port might already be used (e.g. by a permanent USB stick), so that a GPIO solution would be generally more flexible.