Schematics and pcb is designed for use  with i2c or rs232 interface.

Final tested version is I2C version, CPU is powered from I2C side.  (check
ot2smbus2.png).  3.3V zener diode over CPU power pins (D14) is not
recommended. Raspberry PI is used as master device, for i2c. 3.3V from
raspberry is used directly to power CPU.

Other variants designed for using  with optoisolated RS232 (not tested,
check ot2smbus1.png):

1. CPU is powered from opentherm side, 

R4 must be installed, U5 must be installed (low drop 3.3V regulator),
optional R15,R5 resistors must be installed too. No D14 zener diode.

2. No voltage regulator, only D14 zener diode and U5 is replaced by resistor
about 270 Ohms. 

Full version of schematics and PCB can be found in kicad directory


Final PCB files can be found in PCB directory. (.drl and .plt files from
kicad). PCB may be manufactured by cnc isolation lines routing, check .ngc
file (for use with  linuxcnc ... http://www.linuxcnc.org). Check .svg for
isolation line PCB design.


PCB is designed for 1551KFLBK HAMMOND box.