Worked great the first time we tried it. Default was 3.3v and signals were correct polarity.

I bought three DEV-15096 boards (from Digi-Key, along with other parts I ordered there). These CH340 based USB-serial boards are cheaper than my other FTDI/FT232 based boards like the classic Sparkfun DEV-09873, and USB-C is very convenient.

Rx-LED Overloading Issue: Unfortunately this DEV-15096 board has a serious design defect: It ties the Rx LED directly to the Rx input signal line via a pull-up resistor without using any LED-driving buffer. This can overload the serial transmitter on the other end and cause faults, unless that transmitter has additional current sinking ability to drive the Rx LED (in addition to the low current needed to drive RS232-TTL signal lines).

The problem is that certain routers (e.g., Netgear R8000) won't boot up when I am connecting this USB-serial board to the router's serial console to capture the initial boot messages (nothing happens after pressing the router power switch). Tried the other two pieces of DEV-15096 - same thing, router won't boot. Tried two other R8000 routers - same thing. Note that I was connecting only two wires for serial monitoring: A "GND-to-GND" wire, and a second "Rx-to-Tx" wire from the board's Rx to the router's Tx.

The routers boot fine if I temporarily disconnect the Rx-to-Tx wire before powering on the router and reconnect it back a couple of seconds after applying power to the router, and then things work fine - I start getting serial console boot messages (after losing the first a few lines of the initial boot messages).

Importantly, none of my other FTDI based USB-serial boards have any of these problems (the routers boot up fine with DEV-09873 no matter when/how I connect).

So I made some rough DC input impedance measurements for the Rx input pin, both on the FT232 based DEV-09873 and also on this CH340 based DEV-15096 board. Found a huge difference: The DEV-09873's Rx input has an input-Z of ~400 K ohms, while this DEV-15096 had an input-Z in the single digit (K ohms). So a transmitter driving the Rx pin of this DEV-15096 will need to sink a couple of milliamps for a 0V level signal (as opposed to only a couple of microamps when driving the DEV-09873). This is almost certainly why the routers won't boot with this CH340 based board.

Also the schematics show that the FT232 IC has separate special pins for driving the Rx and Tx LEDs and separate dedicated pins for the Rx and Tx signal without any external pull-up resistors/LEDs (and I measured these signal lines to be of high-Z). The DEV-15096 board, if the schematics are right, assumes that the transmitter on the other side is capable of driving the board's Rx LED. That will work in most cases but it seems too much to demand that all RS232/TTL signal transmitters should be capable of driving an LED - some RS232/TTL transmitters may get overloaded by the LED circuit.

The schematics indicate that the FTDI based DEV-09873, although more expensive, have a superior robust, fault-tolerant design compared to this DEV-15096 board. Because the DEV-09873 board's FT232 chip has high input-Z on its Rx input signal line and provides separate pins (with driver/buffer circuit) for the Rx LED, it will not overload the other side transmitter, and so will work in more noisy/demanding/critical situations compared to the DEV-15096 design.

I will go back to using my DEV-09873 type (FT232 based) boards again.

Update: It appears that the culprit is the specific cost-cutting design of this DEV-15096 board (and other CH340 based boards), not the CH340 IC itself. True, the cheaper CH340 IC does not include separate driver/buffers for driving LEDs like the FT232 does, but one can overcome that shortcoming in a CH340 based board design by adding a single buffer gate for driving the Rx LED. In fact, SparkFun's DEV-14050 (CH340 based serial basic breakout) is exactly such a board, whose schematics clearly show that the board designer added a "74LVC1G08SE Buffer for RX LED". So with that buffer for the Rx LED added, the CH340 based DEV-14050 board does not suffer from the Rx-LED overloading issue, and is still significantly cheaper than any FT232 based board.

The V3 pin must be connected to 3.3V when 3.3V VCC is selected. When 5V VCC is used, it basically has a LDO built-in to step in down to 3.3V for it's USB logic to work.

Not sure if anyone noticed, but the LED resistors are not matched to the LEDs so the green LED is about half as bright as the yellow LED in operation. Would be nice if matched resistors were installed in the future.

Edit: maybe it's just the LED I have, both drop 1.8V, but the green one has an internal resistance of 1.3kOhm?

Can we get a SparkX version with the Red Squirrel 3V/5V Switch?

How much current can I pull from the 5V line? Is it as much as my Computer/Power brick will give me or is it somehow limited on board? I would like to use this for development but later on leave it on the board for Powering my project.

I think I'm slightly confused. What's the difference between this and an FTDI Basic Breakout?

I really don't understand why sparkfun does not put a decent voltage selector like jumpers or a small switch on their serial breakouts. :(

if this only had a pin jumper instead of solder pads for the voltage selection it would be more useful than the visiport2.