I have a GNS FM9 dongle, which is a RDS receiver that came bundled with a sat-nav, intended to be used as a receiver for TMC traffic reports (which are transmitted as part of the RDS data stream).
These devices have a UART interface but come in various flavors—connectors differ depending on the device the dongle came with, and logic level can be either 3.3V or 5V. The chip seems to operate at 3.3V; the correct logic level for the device can be identified by the presence or absence of extra circuitry on the board. The one I have lacks the level-shifting circuitry, suggesting it operates at 3.3V. It has a USB cable, which is plugged in between a USB power supply and the device, although the device-side data pins on the USB cable carry UART signals, not USB signals (which is why the device-side plug does not carry a USB logo). It dies mean, however, that it operates on a 5V power supply.
In order to control it from a PC, I have bought a FTDI232 breakout board. The board has a jumper to select 3.3 or 5V. Looking at the PCB, the middle jumper pin seems to be connected to the Vcc pin on the pin header, though I don’t know for sure what else is governed by the jumper setting. The chip on the board is an FTDI232RL.
What is the correct jumper setting for this device (3.3V logic level, 5V power supply)?
In particular, what are the TX voltage levels at either jumper settings? Do I risk frying the FM9 by setting the jumper to 5V, or is TX voltage guaranteed to not exceed 3.3V with either jumper configuration?
Or do I need to go for a hacky solution, setting the jumper to 3.3V (for the logic level) but taking Vcc from a different source, not the Vcc pin on the 6-pin header?