You have totally wrong component in your circuit. The BS170 is an N-channel MOSFET, and the dual-mode circuit cannot operate with a MOSFET (because the substrate is usually connected to the source pin in MOSFETs making them asymmetric). The circuit requires a symmetric N-channel FET [Philips1996][Siliconix1997] and in general only such JFETs are available as discrete components. Do not try to substitute the JFET with much more common MOSFETs. • What must be the power rating of the resistors? Also, should I use BF245A, B, or C? There are only negligible losses in the resistors (you cannot draw too much power from the serial port). So the smallest possible resistor power rating is ok for all resistors (for example 0.25 W). There is no difference between BF245A, B, or C in this circuit. • I have problems in FBUS communications with the dual-mode adapter. How to start searching for the reason? To start solving your problem, try to disconnect the D or S of the FET Q1 to make the adapter FBU