Question: I have various devices to connect to my iMac using good-old RS-232.

Of course, I only have USB connectors on my machine.

I have a couple of Prolific PL2303-based cables, and they seem to work OK, but the kext provided seems a little flaky and I'm not sure about long-term support.

What is the most-stable, best-supported USB-Serial chip-set or cable?

Answer: 
I'm professionaly using a
[Roline USB to Serial 0.3 m long adapter](http://shop.roline.com/roline_converter_cable_usb_to_serial_0_3_m/12.02.1089-20.html?t_Action=searchKatalog&t_SearchValue=&t_Sort=&manufacturer=&device=&t_Hier=&t_SubHier=&t_lager=&t_CatID=3078&t_ParentID=3077).
I'm using the [PL2303 OSX driver](http://sourceforge.net/projects/osx-pl2303/).

When I plug my cable, this driver dynamically create `/dev/tty.PL2303-0000nnnn`. I use it from a `Terminal` or `xterm` window with:
`screen /dev/tty.PL2303-0000nnnn`.

0 problem with hundreds of connections on network equipments and servers (Brocade, Cisco, Extreme, Oracle (ex. Sun)…).
Both Prolific PL2303 and FTDI devices work fine.

I'm using a TrendNET TU-S9 v2 currently (PL-2303). Other than the vendor provided driver or driver from Prolific's Taiwan website, options include <https://www.mac-usb-serial.com> (fairly inexpensive driver), and the nice Mac app <https://www.decisivetactics.com/products/serial/> (not free but works well) which has built-in drivers and doesn't require a separate driver installation.

I have also seen claims that FTDI drivers are built into MacOS now but haven't personally tested since I don't have a FTDI device.

One thing to be aware of - to connect to PC's and certain PC-like devices with serial ports (some routers/firewalls etc) you need a null modem adaptor or null modem cable between your USB-serial device and the PC/device you're connecting to, usually with female-female connectors.