For reference, is the USB plugged in to the non-Jetson computer? This is what I have been going by.
Next, the USB end would show up as “
/dev/USB0”, but there is also a non-USB end which is wired to the other UART. The non-USB end must be a 3.3V logic level, and technically the RS-232 standard is a wide range of voltages via a DB-9 (technically, this is a “DE-9”, but DB-9 became the common terminology) connector. I need to make sure we are talking about wire pins at 3.3V and not an actual DB-9 connector since DB-9 changes the PHY voltages. Note that if the UART side is not 3.3V, then it won’t function entirely correctly on a Jetson, and if the RPi or other units are some other voltage, e.g., 1.8V or 5V, then it would still function correctly on those.
Regardless, both sides will have a TX and RX pin. Wiring a TX to a connector’s own RX (and possibly also CTS and RTS if CTS/RTS flow control is used) would create a loopback connection good for testing. If you connect two separate UARTs, then there may be subtle differences, e.g., one end using a different character set than the other, or one end wanting flow control, but not the other end, and this could result in differences in how the two sends actually work. When in loopback you are guaranteed that a device will correctly use its own same character set or flow control settings. Both ends would also be guaranteed to use the same logic level voltages. Thus it is quite useful to first validate each end can talk to itself, and only then move on to two separate devices talking to each other. If there is a problem in loopback mode, then the problem is significant. If the same loopback working UARTs are then attached to each other rather than loopback, and something then goes wrong, it can be inferred that there is a problem in either software settings or voltage of logic levels.
Is this a 3.3V logic level? Does loopback work at each end, or is any device unable to speak with itself? In the case of loopback, is it necessary at either side to wire CTS to RTS, or can both operate normally without CTS/RTS wired?