For those wanting RS-232 expansion card, I’m placing my entry in the hat.
This expansion ard is based on a classic MAX232 transceiver for RS-232 with MCP2221 doing the UART to USB duties. The final RS-232 signal ends up being around ±8V which is pretty much standard for such setup and well within specification (±3-15V).
Card is of non-isolated so all ground loop warnings apply. However, considering vast majority of existing USB to RS-232 cables are non-isolated too, this is nothing new or unexpected. Essentially, if a standard converter works for you, this expansion card will work too.
Card connector is a shrouded 3-pin JST XH header which also allows you to use standard 0.1" jumper cables for testing. For a more permanent setup, a cable with JST XH connector is of course preferred (many such precrimped cables are available on eBay/Amazon).
While I do offer a hand-assembled devices for purchase, the whole project is also available on GitHub so you can roll your own.
A bit more details can be found in my blog post.
This looks great!
Any reason why you forewent exposing any of the other RS232 signals?
Mostly to keep it friendly for hand soldering.
If I wanted to expose the additional signals, I would have needed to switch to another UART converter with a higher leg count. Since I wanted to skip dealing with an external crystal, that would leave me with FT232RL as a most likely candidate and I really try to avoid that one.
Further more, I would need a bigger RS232 transceiver handling all those signals (with of course more pins; probably SSOP) and larger size caps. Combine that with bigger connector and everything becomes really crowded. Not a deal breaker, but definitely more annoying to solder.
Also, due to a connector size limitations (if I wanted to keep it 0.1" friendly), I could fit 5 pins at most. Taking the mandatory GND, RXD, and TXD, that leaves enough pins for either DSR/DTR or RTS/CTS pair - not both. And it seems there is no clear standard on which pair is more used for flow control. Some devices like DTR/DSR but other devices like RTS/CTS better.
After going over all devices I own that have a RS-232 port - mostly routers and some industrial stuff, I actually found that not one requires the hardware flow control to work. So I opted to keep it simple and stick to just data. Yes, I know they do exist but it seems not to be as common these days as it was back 20 years ago.
I might create a FT232RL based card in the future but at this point it seemed to me those extra pins would just stay unused. Unless I have a strong case for adding extra pins, I prefer to keep it as simple as it gets. Type-C connector is hard enough to deal with.
This right here is what I love about the community, actual polite discord about design. Someone asks a question and gets an actual answer back not just a quip.