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: 
Just to be clear, **I have not used this device myself.**

Stewart Cheshire, creator of ZeroConf (the basis of Bonjour), gave a Google Tech Talk on the subject of ZeroConf, and presented a couple of embedded devices (Cameras, and an RS-232 unit).

I've always wanted to try this out, but have never had a good enough use to cough up the money to do so.

Stewart presented an RS-232 over Ethernet module from [SitePlayer](http://netmedia.com/siteplayer/telnet/index.html). They have a built in web interface used to set up the ethernet and serial parameters, and if I understand correctly, you simply telnet to the device's IP address in order to be presented with the serial interface. (See their [pdf](http://netmedia.com/siteplayer/telnet/documents/SitePlayer%20Telnet.pdf) on the subject.)
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.