openinverter forum

Issue:
After connecting to the ESP8266 through the web interface I get the error "Connection problem between ESP and STM".

Precursor:
So I purchased a Zombieverter from evbmw.com, specifically the 350 Euro version. I received the board and the 2024/08/03 parts package. I soldered on the L4 and L5 components, tested with a multimeter that they were both soldered in using the test points. I made sure they were not bridging. Soldered on the headers for the esp8266 to the board and the esp, as well as the enclosure connector. All solder joints are good, and I've also confirmed that the solder joints for components preinstalled are good. The ESP web interface works properly in the sense that I can connect to it over 192.168.4.1 and the Zombieverter 3V3 light is on as well as the flashing ACCTY light. I have confirmed on the zombieverter that the 5v and 3.3v lines are correct.

Extra information:
After testing for a few days, the boards never draw more than 1.5 watts at start or during running. I've tested using either a lab bench power supply or a car battery, hoping it was my power supply. After 2 days of testing, 1 time I turned it on and the Zombieverter DID connect and is confirmed to be on version 2.17a. The web version is also on version 2.0. That ONE time it did connect, the connection between the ESP and STM was very flaky. It would flicker from connected to not every 3 or 4 seconds. when I powered it off and back on, the esp and stm never connected again, like when I first started. At this point I knew it was possible, but after 2 more days of testing voltages and traces, the only issue I found is that when the esp is connected to the zomieverter, the 5v line drops to 3.4-3.8 volts across the G and 5v pins. The esp not on the zombie when powered provides 4.5v over those same pins. While that may be the issue, I have supplied 5v straight to those 5v line and the issue is still there. So, not sure that actually is the issue.

If anyone has any idea for this "Connection problem between ESP and STM" error please let me know. I just got this board to use and currently stuck here.

Have you got one of these? Connect the black cable to GND, green and white to rx and tx (of course I don't know which way around).

Then open a terminal program on your computer with a speed of 115200 baud and at the prompt type and enter.

What do you see?

I have a serial adapter cable on the way, but when I plug in the board over micro B and read from the serial port over 115200 it results in this.
{$lܟ|�$�|�l�c|����{�c�c��og�$og���cp�lrl{$p�g��lĜco�|���c��og�d��d`�g'l`gs���'cd�dp�o�sĜܜ��cn�|�c��g'�d`�og$`or���g#��`s��gb��`�d��|�$`��g�l����o�s��o<�d�l`#��|s�l�'��g�d`��s�l�l��

However, if I connect using the same method at 74880 Baud rate I get,

ets Jan 8 2013,rst cause:2, boot mode:(3,7)

load 0x4010f000, len 3424, room 16
tail 0
chksum 0x2e
load 0x3fff20b8, len 40, room 8
tail 0
chksum 0x2b
csum 0x2b
v0005f950
~ld
rf cal sector: 1020
freq trace enable 0
rf[112] : 0�

This only happens once at boot and then nothing else happens, of course unless I click reset and it does it again.

I can check again with the serial cable when it arrives, but this is what I have right now. If I need to, I can grab an arduino mega and use it as a serial adaptor.
Lmk what you think. It looks like its running an older program though.

That's the boot message from the ESP. Disconnect it for the test

Right after your last message I wrote some code for the arduino mega and read from the esp and got the same thing as previously.
So I waited for the FT232RL and the ST-link to arrive since I figured I was out of my depth there.

They have since arrived, and after using PuTTy at 115200 baud rate, I do have more messages to review.

Connected to the Tx and Rx while the esp is connected to the zombieverter: This is a zip file of 5 boots from powered off
https://drive.google.com/file/d/1Qf5Clf ... sp=sharing

If I swap the Tx and Rx lines on the FT232RL, out of curiosity, while the esp is connected to the zombie verter, on boot it simple only ever prints "2".

If I disconnect the ESP from the zombieverter and only read from the zombieverter, nothing ever happens.

If I only read from the ESP, disconnected from the zombieverter, I only ever get the boot message that is readable from 74880.

After reading, viewtopic.php?t=4969, I decided to check on the resistors on my wemos module.
My module: https://drive.google.com/file/d/1mQsRQY ... sp=sharing
New modules online: https://drive.google.com/file/d/15ejpZ2 ... drive_link

On my module, the 103 resistor on the RST pin is reading 5.5k ohm instead of 10k ohm. Additionally, I noticed that between the 2 images, there is a chip missing from my wemos module next to the reset button. In the other open inverter post, I noticed it mentioned that R6 being 100k ohm was bad and that it should be lowered. The only 100k ohm resistor I see on my board is the 01D resistor on the side of the board next to the micro B header. After checking schematics of this board I have realized there are a lot of variants of it and just wanted to check this 01D resistor was the one R6 was referring to.

As of this moment the connection problem between ESP and STM error is still present.

The Wemos seems unhappy, can't tell why.

Seeing the "2" coming from the STM32 once sounds good. It is the "hello" message from the boot loader.

With your serial cable connected in the way you see the "2" and the Wemos disconnected - what happens when you type "json" and press enter?

Set it back up in that way and looks like the parameters list

Text file of the data: https://drive.google.com/file/d/1b0psmp ... sp=sharing

Ok, so the STM32 and the connection up to the Wifi module behaves as it should. So the module is the culprit. Somehow

After a few more hours of tests, I have to agree.

I have gotten it to connect once today, and it was very stable. Of course, after a reboot it broke again.
To make sure it was not a power fluctuation issue, I added a big capacitor to the 3.3v line and that fixed the rst cause: 4 issues that occasionally occurred. Since one of the errors meant possible flash storage issue on esp, I uploaded a basic blink sketch to the esp and it blinked like normal. I then reuploaded the esp-web-interface code and it ran the exact same as before the wipe. So, that wasn't it.

I decided to read from the zombieverter while the esp was connected and got "2" and then some garble. and then I wrote "json" while the esp was still connected and the zombie made the same list as before. BUT, now when I connected to the esp over wifi the error was gone and I was able to edit the parameters over the web interface. After a reboot it didn't happen again. Additionally, trying to repeat the same process of manually adding the word "json" in didn't work since the esp stopped me from entering it in until I disconnected it, and of coarse then it could not read the data because it was disconnected. I guess the first time that it let me was just a fluke of entering it in while the esp wasn't transmitting.

Data output Attempt 1: https://drive.google.com/file/d/1jID2gF ... sp=sharing
Data output Attempt 2: https://drive.google.com/file/d/1a2py8A ... sp=sharing

I'll order a small handful of the wemos d1 mini boards, upload the same code and see if the results differ. I'll let you know what happens.

The new Wemos D1 Mini boards arrived. I uploaded the interface to one, soldered the pins on, and plugged it into the zombieverter. First try it connected and everything is up and running. Installed it into its enclosure and it's working great!

Solution: Buy and program a new wemos D1 mini because the original was being funky

Thank you very much for your help!

For anyone else having this issue as well or needs a full step by step guide on how to set up your pc and upload the code to a new wemos board, I'm making a video on how. There is a lot of information on this forum on how, but in a lot of different locations. So, I figured I'd make it really easy. I'll reply to this thread when it releases.

Other forum posts and websites that are helpful:
Arduino ide: https://www.arduino.cc/en/software
Files for esp8266 board manager: https://github.com/esp8266/Arduino
Web interface code: https://github.com/jsphuebner/esp8266-w ... me-ov-file
Clear upload instructions: viewtopic.php?t=8
For uploading spifs: https://github.com/esp8266/Arduino/blob ... system.rst
Additional filesystem downloader part 1: https://github.com/esp8266/arduino-esp8266fs-plugin
Additional filesystem downloader part 2: https://github.com/earlephilhower/ardui ... efs-plugin
How to upload spifs guide: https://www.youtube.com/watch?app=desktop&v=pfJROpQg-Is
Web interface guide: https://openinverter.org/wiki/Web_Interface#Update
For the FTDI to read and write serial data: https://www.chiark.greenend.org.uk/~sgt ... atest.html
For the older olimex boards: viewtopic.php?t=8&start=125
Other similar issues and solutions: viewtopic.php?p=53024&hilit=update+wemos#p53024, viewtopic.php?t=1756
History of the error message: viewtopic.php?t=4512
Zombieverter releases: https://github.com/damienmaguire/Stm32-vcu/releases

To wrap this up for anyone else that may come across this post looking for help, here is a video guide I made of how to program the esp.

This video includes a small unboxing, troubleshooting, how to find new parts for the boards, how to set up your computer to flash the esp modules, flashing one, and getting it all working.

Project post: viewtopic.php?p=76885#p76885

Great post and video, I Had the same problem too.. turn out my module ESP needed reflowing and that fixed it, was going to buy new ESP.

I'm having this very issue still. ESP will show the network and web interface just fine, when powered alone. But once I plug into the Zombieverter, I get no such connection. I get activity lights and ESP board is powered, but don't know why there isn't a handshake between the Zombie and ESP for me to access the network and web interface. I will reflow the solders on the pins to see if that is an issue and report back.

I'm still having this issue. Even with new ESP and soldering reflow of the pins.

I have installed a new D1, made sure all the solder was solid on the pads, and still no handshake from the Zombie. I can access the network interface when the ESP is powered by itself, but once on the Zombie, and yet I get activity lights on the Zombie board, but I don't get any network transmitting from the ESP for me to connect to the web interface.

What is the next step I need to do? I have limited coding knowledge, so I can't get really deep inside the code, unless there are tutorials that can be pointed to for me to learn.

I'm assuming this maybe on the Zombie side of things. I'm scour this forum, but if anyone could help direct me for the next steps on how I can correct this somehow?

The board was directly purchased from Damien and should be pre-programed, I would hope.

Can you measure voltage on the D1 when connected to Zombie?

And a picture might be helpful

I was able to solve the issue, I found out that I had a connection between TX and RX that some solder flowed on the underside of the pins that caused a short. I cleaned and put in new pins. Everything is good. It was going good. I had a connection and everything, but I made a huge mistake. While I was uploading the new bin file for the Zombieverter, I forgot to take my wifi off of "connect automatically" and it switched to my wifi network while it was uploaded and bricked the Zombie. So I have to reflash it. Poor luck on my part.

I had an ST-Link (believe it was a clone), but was able to reflash the Zombieverter. Took a bit of finagling, but it's all good, connecting and showing the latest 2.30 update.

Thank Gawd! Now it's time for some serious parameter settings.

I am dealing with issues with mine as well. I have some random ESP8266 and I honestly don't know which one to get. Non the less I feel I am close. I followed the settings in the video above but I see the readme in Arduino says to use different settings.

Ok to make sure I am clear. I have mine working but it has issues. It works for a bit then gives the warning "connection problem between ESP and STM." I guess this is more likely a problem with RX and TX? Seems a LOT of people are having this problem.

Hi I have made progress. But as you can see in the video it doesn't seem like a great connection its usable but barely and never works on my phone. Anyone have any ideas on how to improve this?

One of the biggest problem I have ever had with wifi on Zom and other OI boards is if there is another accesspoint or wifi source in the vicinity on the same channel. You can test this with a wifi scanner app.

Jack Bauer wrote: ↑Wed Oct 22, 2025 2:29 pm One of the biggest problem I have ever had with wifi on Zom and other OI boards is if there is another accesspoint or wifi source in the vicinity on the same channel. You can test this with a wifi scanner app.

So I think the App is showing me its on channel 1 which is conflicting with a ARLO camera one of the neighbors must have?

How do I force the ESP to run on another channel? I have been looking at the INO file don't see anything and I can't find the ESP8266Webserver.h file in my folders to look in it....

I'm not too sure but you can try this:

In the esp8266-web-interface.ino file of the esp8266-web-interface channel can to be "declared" The syntax is So something like the below *should* work

Thank you. I will look at this when I get home.

Hmm So using Grok I was able to add to the code to get it to change channels. But I have tried channel 6 11 and 13 as well as 1 and nothing is working very good. IN fact its all very Unusable. I am not sure what to do. Is there another way to set up the VCU? Can I make a USB connection or use a TTYL adapter or something?