Trinket + SoftwareSerial: Communication only if FTDI cable plugged in

Hello Everyone,

I'm trying to set up a communication between my Adafruit Trinket Pro 5V and an Arduino Uno.
Due to the structure of my board I need to reconfigure the Serial pins via SoftwareSerial so I can use the original pins for something else.

For the slave I call:

SoftwareSerial mySerial(A5, A4); // RX, TX
//Use original Serial Pins as Input
pinMode(0,INPUT);
pinMode(1,INPUT);

For the Uno I call:

SoftwareSerial mySerial(5, 6); // RX, TX

I wired RX-TX accordingly and connected both the Trinket and the Arduino Uno to a common GND.
I also have a diode and pullup in place because I want to use multiple slaves later on. (see Image Tutorial)
I'm programming the Trinket via FTDI cable because I find it way more comfortable.

Now to the real problem:
The communication works fine. I can recieve the data with the arduino Uno and display it via the Serial Monitor.
But as soon as I unplug the FTDI cable, the transmition stops.
The remaining functions of the Trinket however stay intact (LEDs, Sensors).

I assume this is due to some fancy internal wiring of the FTDI-Serial Header I do not yet understand.

Does anyone have an idea what is causing this?

Thanks and best regards!

Post some photos, a schematic, and a link to that fancy USB adapter.

I'm using this FTDI cable with integrated IC: https://www.amazon.de/gp/product/B083HVM7VZ/ref=ppx_yo_dt_b_asin_title_o09_s01?ie=UTF8&psc=1
I attached the scematics of the main unit with the Uno as well as the trinket.
The sketch does not show the pullup on the serial.

Does SoftwareSerial really entierely reconfigure the Serial Interface or do the Pull-Down Resistors on the original TX/RX pins corrupt the communication?

SOLVED!
The GND connector between the two boards had a loose contact.
Without a common GND the serial communication is corrupted and wont work anymore.
Due to the USB cables providing the common GND instead of the direct connection it worked when plugged in that way.

Thanks four your hints anyway!
Best reagards!!!

Thanks for the update! Glad you solved the problem.

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