How to connect RS232 level shifter?

Question

I'm trying to use this:

https://www.sparkfun.com/products/449

to connect my RPi to my computer via a USB to serial device. (And eventually connect the RPi to other legacy serial devices)

My questions:

1. How do I determine what voltage the USB to serial device is running at? Does it matter in this case?

2. Assume the USB to serial device I have runs at 5V. Do I solder the Vcc pin of the PRT-00449 to 5V on the Pi or 3.3V? I was originally assuming 3.3V (which would imply that I could then use serial devices of any voltage, negating the need for question 1) until I found this page:

https://www.abelectronics.co.uk/kb/article/1035/serial-port-setup-in-raspberry-pi-os

"Connecting a 5V device to the UART port can damage the Raspberry Pi GPIO header."

I'm assuming that the above page is implying "unless you use a level shifter" but rather than risk damage, I'm asking here.

3. How do I connect RX and TX of the PRT-00449 to the Pi? Does RX of the PRT-00449 go to pin 10 on the RPi? (UART RX) or to pin 8? (UART TX)

I know these are very basic questions but it is very easy to get this backwards, risking damage to my devices, so having this explicitly defined is extremely helpful.

Answer 1

How do I determine what voltage the USB to serial device is running at? Does it matter in this case?

No it doesn’t matter. The adapter is powered from the USB side. If you look at the schematic of the Sparkfun board you will see the power is not passed through the serial connector.

Assume the USB to serial device I have runs at 5V. Do I solder the Vcc pin of the PRT-00449 to 5V on the Pi or 3.3V? I was originally assuming 3.3V (which would imply that I could then use serial devices of any voltage, negating the need for question 1) until I found this page:

Use 3.3V or you will risk frying your GPIOs.

How do I connect RX and TX of the PRT-00449 to the Pi? Does RX of the PRT-00449 go to pin 10 on the RPi? (UART RX) or to pin 8? (UART TX)

Connect TX on the Pi to RX on the Sparkfun board and RX to TX. There’s an example connection diagram here.

https://github.com/SparkfunTechSupport/Hookup-diagrams/blob/master/PRT-00449/PRT-00449%20Using%20with%20external%20power.jpg

Good luck!