That is the serial interface manual for all(?) TNCs, I referenced it (page 43/100) to check if I got the machine parameters right, the same information is mentioned in the post #6 in this thread (I actually found in my storage a db9-db25 null modem cable that had this exact routing), I also tried the non-block transfer mode (MP223: 0), and various other settings I could find googling for TNC RS232 issues people had, but nothing in the machine parameters suggests that it may affect the signal level, so I concluded my problem to be hardware related, not the machine parameter settings, in fact, the hardware interface manual page 6/100 shows a diagram of the expected signal levels, I attached that picture showing how signals look on scope on my control, red one is data sent from PC to TNC, blue one TNC>PC, and when I use that cable in between two PCs, then signals in both directions have the amplitude of the red waveform