So I have a headless NAS that I'm trying to connect to via a serial console so I can install a different OS. It has a 3.5mm jack on the back I/O labeled COM1.
I had a USB->TTL cable I had used for a raspberry pi that has 5V for the power, and 3.3V for the RX and TX pins. I cobbled together a USB-> 3.5mm cable by splicing a 3.5mm aux cable to the TX,RX,GND pins and while I get stuff printed to the console, it's all gibberish. I've read that this is usually due to an incorrect baud rate, but after checking the serial port settings on the NAS via an SSH connection, I can confirm that I do have the correct baud rate. I've also messed with all of the other settings like the # of bits, parity, stop, etc and haven't been able to get anything legible.
After posting this question over on the UNIX stackexchange, someone suggested measuring the voltages to make sure it's really TTL vs RS232 levels, and seeking advice over here. The voltage of the 3.5mm port on the NAS gives almost -6V (it usually jumps to -6, then drops to 5.9/5.8), which from reading more about RS232 vs TTL leads me to believe that what I need is RS232 levels since RS232 uses negative voltages and TTL doesn't, am I correct in that assumption?
I came across this question on this site regarding serial cables, and although I'm still fairly confused by all of this, I think I either need a USB->3.5mm RS232 cable (if they exist), or 2 cables: USB->RS232(DB9) and RS232(DB9)->3.5mm.
Am I on the right track with this? I appreciate any help anyone can give. Also, if anyone could recommend a good quality cable too I would be most appreciative, since that's another issue that's come up in my searches.
I can also provide pictures of the board if needed. It's a QNAP TS-963X and unfortunately I haven't been able to find any information or schematics or anything on the motherboard online. Searching what appears to be serial or model numbers don't yield anything relevant.
Best Answer
Based on the voltages being +/- 6V, you need a USB to serial cable that uses RS232 levels (not TTL) and 3.5mm plug. There are several ways you can achieve that, which might include adding RS232 tranceiver to your TTL interface, or direct USB-RS232 adapter that has one buit it. The remaining task is to interface the right connector.