RAK5860 / RAK12500 undocumented conflict

I have a RAK19007 using a RAK11200 core with the RAK5860 BG77 modem and RAK12500 Ublox GPS and have an issue that if both the modem and GPS are turned on, my serial connection gets corrupted by what I expect are the 2 units trying to talk on it at the same time with different rates.

Ideally I want (or rather can only use based on the doco) serial/uart for the modem and i2c for the GPS. I have the modem in the IO Slot and GPS in Slot D.

I tested both modules separately and they work as expected. However, having them both on the board and powered on causes issues. I looked through all the documentation on the RAK site and used the Pin Mapper spreadsheet and there are no listed conflicts, however, it does seem they share the Serial connection.

Also, it would seem the WB_IO2 pin is also shared, in that it enables the Ublox GPS but also the antenna for the Quectel modem (which turns its GPS on). Again, the Pin Mapper does not show any issues.

I would be really grateful for any insight if I am doing something wrong (very possible), and how I can get these 2 modules working together.


Welcome to RAK forum @NathanAus ,

As I understand it, there are two issues you encounter:

  1. UART conflict
  2. IO2 enabling supply 3V3_S.

Here’s my thoughts,

  1. You can disconnect the RX and TX on the RAK12500 so it will not conflict on the RAK5860. You have to remove this resistors though,


  1. I am not sure what’s wrong on this one but IO2 controlled is tied on enabling the GPS module as well as the cellular antenna of RAK5860. There is no resistor we can remove to disconnect them.

Btw, I’ve looked at the pin mapper and it seems to show the possible conflict on UART.


Hi @carlrowan , thanks for the quick response.

In regards to 1. Is there anyway to achieve without removing the resistors, maybe using another slot? Or is it possible to order units without the Serial connected on the RAK12500? Would a different WisBase work?

For 2. Just wanted to clarify that you are confirming, the same pin (IO2) does turn both on?

Also I just downloaded the latest Pin Mapper from your site to double confirm, but I do not get the conflict like you show? Also, on the UART it shows NC, I thought that means Not Connected?

Hi @NathanAus ,

I figured out the difference why the conflict is highlighted on my end. I selected RAK4631 as core while you set RAK11200. I do not think there is another way than removing the resistor. You can try to use other slot (I2C is present in all slot) but it wont fit the way how it should be since the RAK12500 is a long module.

IO2 only controls the active antenna of the cellular module (RAK5860) but on the RAK12500, IO2 controls it voltage supply.

Thanks @carlrowan, I appreciate the responses. It’s a shame that I have hit this issue, we have been loving your boards and were making some great strides in our development, but this is a bit of a wall to hit now and not sure how we can proceed.

I thought I would just close this off by saying I was able to get both modules going by accessing the GPS over I2C and disabling comms over UART (which was an undocumented flag). Then turning on the modem and disabling the GPS on that with an AT command. So both still have access to the same serial but only the modem’s cellular commands will ever talk over it.