Tiny rtk - uart tx power

i have the following issue:
tiny rtk modules cannot pull down rx pin from arduinos or 3ds telemetry modules.
scope shows 3v3 level with minor peaks when module transmits. when not connected the signal looks good (0/3V) . software serial with high impendance seems to work.
Donwe need buffers for serial connections or can we increase current on tx pin?

UART communication using two different voltage levels is not safe.

  • TX (5v) -> RX (3.3v): OK
  • TX (5v) -> RX (5v): OK
  • TX (3.3v) -> RX (3.3v): OK
  • TX (3,3v) -> RX (5v): BAD.

You really need the communication voltage converter.

For TX (5v) -> RX (3.3v):
Use bridge circuit with 2 resistors to switch from 5v to 3.3v.

For TX (3,3v) -> RX (5v):

Use MOSFET as shown below.

And better yet, you need to buy a module like this: