Celejar wrote:
These adaptors are 'smart', i.e. they contain electronics to do the conversion, so as with hardware generally with linux, what counts is the chipset. I believe that many of these devices use the Prolific PL2303 chipset, which shows up with 'lsusb', at least in my case, as "ID 067b:2303 Prolific Technology, Inc. PL2303 Serial Port". This chipset has kernel support via the pl2303 driver. I have another adaptor whose ID apparently isn't in the database; nothing shows up other than the numerical ID. It is also grabbed by pl2303 (although IIANM, that wasn't the case several months ago, when I first acquired it), but it doesn't seem to be working properly, as opposed to the Prolific one, which works fine. I purchased both my adaptors on Ebay. They are virtually identical in appearance, except for color; both have attractive silver cables sheathed in clear plastic, but the Prolific has black plastic covering the two cable ends (serial and USB), while the other has translucent green. This may not help you. I originally bought these adaptors to use with serial modems, but I was never able to get them to work properly, and I never figured out why. I'm currently using the Prolific to manage a headless box via a serial (null modem) cable, and it works perfectly. The other one is not working properly, as above.
Unfortunately, 'serial', and even 'RS232' (which *only* specifies voltage levels) are not one but a whole family of communication methods. While the send and receive pulse structure is fairly standard, the handshake methods are not. 'Proper' serial ports with 25 pins have a range of old-style modem control and handshake lines, of which a few are implemented in the 9-pin version.
There are no real standards as to which control lines are used for which purposes, or which control lines they are connected to at the other end. Sometimes, connector pins at either end are linked together, and often do not connect to the other equipment at all. That's why two pieces of equipment can work together with some cables and not with others. It's not that the non-working cables are necessarily faulty, they may just be wired differently. There's no such thing as a 'standard' serial cable.
Also, serial ports may not be used in the expected way. Sometimes the control signals are used alone, without transmit or receive lines being involved. Older UPS equipment used serial cables to tell the computer to shut down when there were power problems, and usually only one or two of the control lines were used, not the data lines.
And here's where we get to the point: while all USB-serial adaptors will handle the data correctly, they don't all manage the control lines in the same way as a real serial port. I have a serial-port microcontroller programmer which nearly works using a USB adaptor, but it's unreliable and occasionally wrecks a microcontroller. This is because the programmer and its software make use of the control lines to switch voltages. It works perfectly on a proper serial port, even a nine-pin one.
So there's no way at all to predict whether a particular piece of equipment, or particular software, will work with any or all USB-serial ports. Probably it will, but the only way to be sure is to try it.