I have a device, namely a Synology DiskStation. It has a 3.3V RS232 interface on board. All I have is a 5V RS232 to USB adapter. Can I attach it to my DiskStation without making any smoke?
I'm not an electronics expert but everything I've ever read about attaching serial ports to boards that have 3.3v UART pads says to use a TTL converter, or convert the voltage somehow. So I believe you really need something to convert to the proper voltage.
You might get away with it if you only use it to receive serial data from the UART, but I wouldn't chance it.
The RS232 standard specifies +/- 3-15V
If any device is advertised as RS232 and emits smoke when presented with a valid 15V RS232 signal, I'd say it is suffering from a manufacturing defect which the manufacturer should rectify at no cost to you.
"RS-232 drivers and receivers must be able to withstand indefinite short circuit to ground or to any voltage level up to ±25 volts." - Wikipedia
You could always ask the manufacturer?
The serial port on a Synology NAS is not RS232.
"The serial port on Synology NAS boxes is using 3.3V TTL levels, which have to be converted into regular RS232 levels by a level shifter circuit." - NetBSD