Pro Mini 3.3v <-> ESP8266 Software Serial voltage drop on Pro mini Rx

I have a Pro mini 3.3v 8 Mhz hooked up to ESP8266 (Wemos D1 Mini), both are powered separately through different USB ports, the Pro Mini is powered through FTDI adapter with jumper selected on 5v (connected to Raw pin on arduino).

I am using Software Serial 19200 baud rate (which isn’t max), the problem is:

Pro Mini Tx → ESP8266 Rx is fine, sending always and receiving correctly
ESP8266 Tx-> Pro Mini Rx is not stable, i got one clear message every random sends

connecting to oscilloscope i noticed that the voltage level on ESP8266 Tx-> Pro Mini Rx is around 1.8v while it should be around 3v

the confusing thing is, when i changed the FTDI jumper to 3.3v (still connected to RAW not VCC) it worked, the voltage level was around 2.2v (not perfect but it works), when i disconnect the Pro Mini side, the voltage jumps to 3v again !

screen shots:( blue line is the healthy connection, purple is one having problem)

  1. FTDI at 5v connected to RAW pin at Pro Mini
  2. FTDI at 3.3v connected to RAW pin at Pro Mini
  3. Rx at Pro mini is disconnected

Did you check the power supply voltages on both the Arduino and tge ESP8266? Are their VCC voltages constant 3.3V?
(Also, I'm assuming they are both connected to the same computer, so the grounds are connected, right?)

mohsh86:
I am using Software Serial 19200 baud rate (which isn't max), the problem is:

Pro Mini Tx -> ESP8266 Rx is fine, sending always and receiving correctly
ESP8266 Tx-> Pro Mini Rx is not stable, i got one clear message every random sends

My experience is that Software Serial works stable only up to 9600 baud (sending may be ok with higher rates, but receiving is not). You can try AltSoftSerial.

both are powered separately through different USB ports

This might not be a very good idea, if GND and GND AND Vcc1 and Vcc2 from the two USBs are connected somehow. But to tell more, a schematic wold be necessary.