Different PCBs and modem modules in COM.LTE unit!!

[Senspix]

We have a number of M5Stack CoreS3 ESP32S3 loT Development Kit units, including a DIN base, for a product prototype.
We’ve also purchased 13 COM.LTE units from 2 different, Swedish, distributors.

And we have problems…
See text and image below.

Minor issue but still irritating. The mechanical tolerances of the DIN rail mount are such that it is unusable on all DIN rails we have tested (several). Perhaps DIN-rail specs are different in Sweden…. (probably not though).

Major issue with modem module.

The first 3 COM.LTE units we purchased work fine. I used the TinyGSM library with the ”SIM7600” header/spec. These units have a PCB that ”is aware” of the CoreS3 existence, see image, and also include a SIMCOM SIM7600G-H modem module.

The second set (10) of COM.LTE units ….

• The antenna cable on all units was connected to the contact marked GNSS. This is not correct!
• The PCB is most likely older. No CoreS3 ”marking”.
• The exact same code does not work!

Please note that the pin mapping from modem module UART RX/TX to the bus contact is the same on both PCBs ( I measured/tested) and the DIP-switch setting is the same.
Due to other sensor connections I want to keep the DIP-switch setting => I use GPIO 1 and 7 to communicate with the modem module. SERIAL_AT.begin(115200, SERIAL_8N1, 1, 7, false);

So now I have 10 COM.LTE units that do not work and I really don’t know what to do. Return/throw away/?

Perhaps someone has some tips

BTW1: Yes, I’ve tried miscellaneous modem test code from the TinyGSM library as well as even simpler code to test the UART connection.

BTW2, I’ve ordered 5 new COM.LTE units from distributor that supplied working units. We’ll see how it goes.

[felmue]

Hello @Senspix

I don't think the PCBs are different - to me it looks more like a sticker has been added to the same PCB. But I could be wrong.

Maybe try swapping a SIM7600G (non -H) into a PCB with sticker to see if that works? Also make sure you firmly stack the COM.X to your M5CoreS3. It could be a simple connection issue.

Maybe this simple test program might help evaluating the hardware? It sends an AT command repeatedly. Edit: I just re-read your post. You already tried simpler programs.

Edit: Well, it could also be a power issue. How are you powering your LTE modems? Through the DC jack or from the M5CoreS3?

BTW: I noticed the incorrect antenna connection about 3 years ago. See here.

Same with the SIM7600G vs SIM7600G-H version. At the time (3 years ago) the M5Stack shop talked about CAT4, but was selling the non -H version (which is only CAT1). See here.

Thanks
Felix

[felmue]

Hello @Senspix

I just tried with my SIM7600G (non -H; 3 years old) and M5CoreS3 using aforementioned test program and the modem does react to AT commands just fine after about 14 seconds.

Recently I purchased a separate green board with a SIM7600G-H on it which I use in the same COM.X module. Initially it did not work. Turns out the green board had a layout error - RX and TX needed to be swapped electrically in order to work in the COM.X module.

In conclusion: I can use either SIM7600G or SIM7600G-H with M5CoreS3 just fine.

Thanks
Felix