Thank you for your answer
As I wrote in my previous, I am using PixHawk hardware autopilot (the clone of the PX4, marked as Pixhawk PX4 Autopilot PIX 2.4.8, like this http://image.dhgate.com/0x0/f2/albu/g5/M01/7A/CD/rBVaJFi4Q8KAa1uWAAfCAj141F8883.jpg ) with Plane Firmaware loaded.
As I know APM and PX4 are very different from each other in hardware and, therefore, and in software.
For this reason I believe that I have come to the right place
I use the Mission Planner program as an autopilot firmware programmer, autopilot configuration program and to view the data transmitted from the autopilot using the MAVLink protocol.
That's why I'm wondering why the option to select a pin for receiving an analogue signal of engine RPM data disappeared from the configuration of the autopilot and why this feature was left only in the Beta version of the firmware, but it also does not allow you to select the Type of source data _PIN (type 2 can be specified manually, but the data transfer does not provided anyway).
And, in fact, the main question is how to make PX4 transmit the RPM data I need according to the Mavlink protocol, as it was intended earlier. Very much surprised that no one needs it :(...