No boiler messsages, delay symmetry way off

[qjb]

After completing my Opentherm Gateway I noticed few B messages and a lot of 2 and 4 errors. Running the delay with a closed loop between the X1 and X2 results in these values:

OK1A high-to-low: 10us
OK1A low-to-high: 137us
OK1B high-to-low: 1228us
OK1B low-to-high: 4us

Results for OK1B are the same if I move the opto coupler 2 pins from the center of the board.

I rechecked all the resistor values, these are all fine. What could cause these results?

[qjb]

The diodes and zeners are in the correct position and orientation as well.

[hvxl]

With those measurements it's not surprising that the messages from the boiler are not coming through. The fall time exceeds a full bit time! This sounds like there might be a connectivity problem with either R2 or R4.

Your description suggests that your OK1 is in a socket. Can you pull it out and measure the resistance between pin 3 and 4 (should be 100 Ohm) and also between pin 2 and 5 (should be 330 Ohm). Also make sure that you have the correct type of opto-coupler, although the measurements on OK1A indicate that the long fall time isn't typical for the device.

[qjb]

Oeps. With daylight I see the light brown was actually yellow... I had put in 330k. After correcting it the delay symmetry is fixed:

OK1A high-to-low: 19us
OK1A low-to-high: 11us
OK1B high-to-low: 4us
OK1B low-to-high: 6us

Thank you very much!

[hvxl]

I'm glad that's resolved. Those figures look much better.

I would also like to express my compliments on your original problem report. You obviously studied the available information, performed the relevant investigatory steps and reported the result clearly. This helped me to immediately point to the suspect components. Life would be so much easier if all problem reports were like that.