It used to be that devices with RS-232 had negative supplies available, and +-12V were common levels seen.

Later people started using transceiver chips and a 7660 or similar capacitor charge pump to provide a negative supply.

Later still, people started using MAX232 transceivers with integral charge pumps.

Finally, we have effectively TTL level 5V/0V interfaces that can’t drive a negative voltage at all, but might rely on protection diodes and/or resistors on receive lines. Even those, have sometimes become implementations that are 3.3V logic that is 5V tolerant.

When mixing new PCs with old equipment, the inability to drive a line negative for a zero from the PC side can certainly create problems. One would hope the implementation at least accepts the full input range with no damage to the port even if it doesn’t work.

If you’re in a hurry and have a well stocked junk box, I’d suggest building an inline adapter to shift the modern PC levels to a +-12V interface. If time is more precious than money, then buy something already built.

You should be able to connect an oscilloscope to see what’s expected.

I’ve seen devices that just wouldn’t work without driving a line below -3V for a zero and more than +3V for a one.

I’ve also run into things like GPRS modem boards that required 5V TTL levels from an Arduino microcontroller, and using a 5V “1” was required and a 5V tolerant 3.3V microcontroller just didn’t work. For that, the fix was easy…cut a couple traces and glue a TXS0104 based level shifter to the board ?with the VIO pin supplying whatever voltage was expected on the microcontroller side.

Also noteworthy: some industrial single board computers may have serial ports with buffers built in, or the ones on an optional add-on daughtercard that stacks on top might have them where the onboard ports don’t.