Help with USB-TTL converters

I have two Xenons that are wired to a computer using USB to TTL converters both configured for 3.3V operation. The indepedent units are powered by a 12V-5V converter thru their VUSB pin.

One unit uses a generic CH340 converter, and that one works flawlessly.

The other Xenon uses a genuine FTDI cable. On this unit, if I recycle power to the Xenon, the tiny white LCD flashes once but the Xenon never starts. I checked VUSB voltage and it is 5V.

The only way to get the second unit started is to disconnect the FTDI cable from the computer then reconnect it after the unit is powered.

As my knowledge of electronics is limited, I would appreciate the community’s input as to what is going on. I realize a simple solution is to switch to a CH340 solution but FTDI cables are supposedly the standard.

Can you detail all of your wiring, which lines are connected from the FTDI to which pins of the Xenon?

Thank you @dougal.

The FTDI lines are connected as follows:

  1. Black to GND on Xenon.
  2. Red is not connected to anything.
  3. Comm lines connected to RX and TX.

BTW, When the Xenon is powered, the FTDI cable works fine. I also noticed that when I disconnect power to he Xenon (but with the FTDI cable connected to the computer), I am getting a voltage between GND and EN of 1.8V.

I also have a TTL sensor which is connected to VUSB, GND and D5.

Your insight is appreciated and thanks in advance.


Not sure if this is helpful, but I found out if I disconnected only the FTDI - TX wire, the micro does restart.

what is the FTDI cable adapter you are using?

I am using the TTL-232RG-VSW3V3-WE

if you have not already , you might want to get the latest drivers from FTDI website.

What LCD where on what?
Could it be that the EN pin on your Xenon is held low?
You could try connecting back your TX and hook the red wire (providing that’s the 3.3V supply) of the FTDI cable via a 1k resistor to EN.

The issue here is that your getting feedback through the TX line from the FTDI cable. The FTDI cable your using has an internal 3.3v regulator that is powering the VCCIO pin of the FT232 IC. Which means that when you power down your Xenon, the FT232 IC is still powered on VIA USB, and giving your Xenon a bit of juice through the TX line.

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Thank you @seulater.

This is most likely the case. How can I fix this?

Thank you @ScruffR.

I tried your recommendation but it did not work. BTW, I was referring to the CHG tiny LED.

I was skeptical the issue is related to the cable’s VCC because VCC of the other rig (CH340) is also not connected but I am not getting the same behavior.

So “most likely” it has to do with the two FTDI wires that are connected (RX and TX).

I was also wondering if this link is related:

I would appreciate your insights and thanks again for your time.

Hello @ScruffR

Would placing a 20K resistor between the TX line from the FTDI help?

Would any of these 2 circuits work (or something similar)


Thanks in advance.

Depending on the baud rate 20k wont work it far to high.
What he should do is stick a fet inline for the TX signal and drive the gave from +3.3v with a 4.7k pulldown to gnd. When the 3.3v drops, the fet wont let any flow to go through from the FTDI to his board.

Thank you @seulater.

Can you draw the circuit as I have little knowledge of this?

Thanks again for your help.

I just heard back from FTDI that the cable I am using is limited to 50 mA. My sensor draws over 100mA so that may be the cause however I am not connecting the cable’s VCC to the Xenon so I do not know why this would matter?

FTDI recommended I use their TTL-232RG-VREG3V3-WE cable which can handle up to 250mA so I will try this too (in addition to using a MOSFET as a switch on the TX line).