This board can be used to simulate a M-Bus master device instead of connecting to the real AMS unit, i.e. usable for test and development. It takes a 5 V TTL level input (serial) signal and converts it to a M-bus level signal. It supports both sending and receiving. Inspired by MBus_USB.pdf¹, although significantly improved by using zener diodes so that it is flexible and can work with any external power source between 21 and 42 Volt. Most DC-DC boost/step-up converter modules² should be fine for this.

BOM

1 x zener diode 13V (note below)
1 x zener diode 4.7V
1 x BD136 transistor (most pnp bjt will probably work (with 40+ operating voltage))
4 x BC337 transistor (probably any npn bjt will do (with 40+ operating voltage)
1 x resistor 82 ohm
1 x resistor 1k
1 x resistor 8.2k (you can use 10k if power supply is well above 21V)
5 x resistor 10k
1 x resistor 220k
4 x pin header 1x2

Note for the 13V zener. This board design depends on the zener to operate below 1mA, e.g. like 1N5350 (figure 8) whereas a BZX55 (figure 9) will maybe not operate properly.

This limitation could be lifted by reducing the 8.2k resistor so that it draws enough current.

Schematic

SVG version

PCB

SVG version SVG version

Version history

Rev A - Only transmit. Wrong design, does not work. Rev B - Both transmit and receive. Should work (not tested yet).

² Like for instance this one for £3.