Serial [FreeBSD WIKI]

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===== USB to UART/TTL adapters =====

These are common to use for serial access and debugging on embedded devices and single board computers (SBC). They usually come in two variants, as a preassembled cable with dupont/jumper wires at the end, or as a small board with solder points or pre-installed headers.

Common brands of controllers are Cypress, FTDI, Prolific, Silicon Labs and %%WinChipHead%%. Except for evaluation boards / cables they aren't usually the manufacturer of the finished product.

Driver support is available for most FTDI, Prolific, Silicon Labs and %%WinChipHead%% controllers. However, depending on your application, some variants might be of more use than others.

Older controllers might not be able to handle speeds above 1Mbaud, which some devices or %%SoCs%% (such as Rockchip RK3399) utilize, so make sure to verify if that's of importance. In general the difference in price is small so if you're looking to aquire a new adapter you might as well go for one that supports higher speeds.

Controllers that support speeds above 1Mbaud:

  * FTDI FT230(X*), FT231(X*) and FT232-series (excluding BQ or BL variants) work well and they all support up to 3Mbaud \\ 
  * Prolific PL2303%%**%% series  \\ 
  * Silicon Labs CP2102N (only N variant) and CP2104 \\ 
  * %%WinChipHead%% CH340* or CH341* \\ 

Controllers that only support speeds below 1Mbaud:
  * FTDI FT232BQ and FT232BL (these are very old chips unlikely to be found in modern cables)
  * Silicon Labs CP2102 (non N)

===== Buying a serial adapter =====

Pricing is usually around 4-20 USD depending on controller and if you want a value model Prolific and Silicon Labs are usually cheaper than FTDI based variants. TTL interfaces uses either 1.8V, 3.3V or 5V and you **will damage** your device and/or controller by using the wrong one, some are hardset and some have a toggle to switch between voltages (usually 3.3V and 5V).

A good value controller is Silicon Labs CP2104 (soon %%EoL%%) which can be had for about 4 USD. (Products below are not verified in any way) \\ 
http://www.firefly.store/goods.php?id=32 \\ 
https://www.adafruit.com/product/3309 \\ 
https://www.tindie.com/products/makerfabs/cp2104-usb-to-serial-converter-arduino-programmer/ \\ 

Another controller that works well and is relatively inexpensive is Prolific PL2303**TA**, however these are EOL (end of 2020) and there are some reports of counterfeit parts.

Be aware that both **FTDI FT232R/RL and Prolific PL2303HX/HXA** (EOL, look for something else) occasionally have been counterfeit so you might want to consider looking at another controller (model) unless you have a reliable supplier and price seems reasonable (a FTDI chip alone is around 3-4 USD).

Some bugs found in Silicon Labs 2102N
https://www.eevblog.com/forum/projects/psa-do-not-use-the-cp2102n-usb-to-serial-converter-in-your-designs-(for-linux)/

You might also want to avoid %%WinChipHead%% CH340/CH341 given the comments in the driver
https://cgit.freebsd.org/src/tree/sys/dev/usb/serial/uchcom.c#n63

===== Usage =====

Usually they're true "plug 'n play" devices, in most cases you only connect TX(D), RX(D) and GND, VCC is left unconnected.