How can I know whether a random "known not TTL" RS232 device will work with any old random USB-RS232 converter?
There are some cables that convert directly from USB to RS-232, all of these should say so and all should be (reasonably) compliant with the RS-232 specification.

However, there are also lots of cables that translate from USB to TTL asynchronous serial data, and these will be rated at either 3.3V or 5.0V. Such a cable needs a separate TTL-to-RS-232 converter such as the old MAX232 chip.

This is where the confusion begins &mdash; many people call any form of asynchronous serial data "RS-232", even though that term only properly applies to the electrical interface standard. You need to pay attention to exactly what the seller is saying about his cables.

One clue is that if the cable has a D-sub connector (DE-9 or DB-25), it probably really is RS-232. If it has a rectangular header connector, it's probably TTL. YMMV.

The term UART refers to the hardware device that generates and receives asynchronous serial data. Technically, the USB-to-TTL cable contains a chip that comprises two interfaces: a USB device interface and a UART. The chip passes data in both directions between these two interfaces.