alt Hacker NewsFTDI (the company practically synonymous with TTL serial adapters) uses 3.5mm tip-ring-sleeve connectors for this. In other words, a common headphone jack.
I added one to my single-board computer enclosure, following FTDI's wiring. Now I can easily connect whenever I need to use the serial console, and a standard 3.5mm audio extension cable will let me reach across the room without moving my main computer. Replacement parts, if I ever need them, are cheap and easy to find.
Here's the pinout:
https://www.ftdichip.com/Support/Documents/DataSheets/Cables...
Replies
Every vendor defines their audio jack connector serial port differently. It is very dangerous to use 3.5mm jack. There is no pinout standard of using 3.5mm.
Even as pure audio jack, the 3.5mm connector has two standards, with the difference on ground and mic.
Funny - in the retail industry, we use a 1/4" jack instead that runs straight RS232 level traffic (for "DEX" - data exchange - typically sending invoices to a merchant from a route driver, or pulling data from vending machines on a route).
........ and the DEX pinout has Rx/Tx in reverse (our tip receives and our ring transmits) in comparison to the FTDI TTL setup. Wellp!
Using a male TRS 3.5mm audio connector is something you should do with care. When you hotplug that connector the plug's tip (T) may first make contact with the sleeve (S) of the receptible, then briefly hit the ring (R) until it finally lands on the tip connector. Same goes for the plug ring (R) which may have contact with the sleeve (S) of the receptible.
So expected are:
T → T
R → R
S → S
T → S
T → R
R → S
On audio systems this is typically not an issue, since tip and ring carry signals that on any gear worth anything should have an output stage that can easily handle a connection to ground.
That really sucks for hotplugging since TRS connectors sliding in basically make random connections before seating properly.
Granted, you shouldn't hotplug TTL serial, but everyone™ does it anyway. (In some situations you're even forced to, to avoid reverse powering something.)