Optocoupler Adapters: Troubleshooting & Solutions

Hello to all VAGCOM users.

Optocouplers in VAGCOM adapters can, due to their relatively low slew rates on the RXD receive pin of the diagnostic PC, cause signal delays of approximately 20 picoseconds and bit length errors of up to 10 microseconds, even when the adapter is optimally adjusted, depending on the electrical characteristics of the COM port.

Furthermore, the voltage received at the COM port fluctuates by approximately 10 µsec within an undefined range of -3 to +3 volts during each transition from a high (H) to a low (L) state, or vice versa.

"Despite this, the communication via the 2.4 GHz adapter works perfectly fine with my tractor, even when I intentionally exaggerate the errors of the adapter by significantly misaligning the trimmers."

Apparently, VAGCOM can tolerate errors of this magnitude.
On the other hand, this doesn't necessarily apply to every vehicle.

Therefore, I have revised circuit 2.3 and, among other things, added a transistor output stage on the PC side, which reduces the execution times to below 10 µsec and the bit length error rate to below 2 µsec.
The slew rate is comparable to circuits without optocouplers, and it reduces the time that is not defined as high (H) or low (L) during each transition to approximately 0.5 µsec.
Unfortunately, this circuit has approximately 15 more components than versions 2.3 and 2.4, and I'm wondering if there are situations where this extra complexity would be justified – perhaps in PC-car setups where all previous opto-based solutions have failed.

I would therefore like to know if communication problems still occur with adapter versions 2.3 or 2.4 (or updated older versions - see the link in the adapter article), while adapters without optocouplers (Maik's version or newer purchased parts) work without issues.

If there is a need, I would email Rainer a version 2.5 for him to upload to the diagnostics page, for the proponents of the Opto principle (if there are any others besides me ).

Has anyone actually measured the timing differences between the TXD and K-Line, or between K and RXD, on Maik's adapter, using traditional methods like a square wave generator and a two-channel oscilloscope?
I would be interested in seeing the data for comparison with Opto-Adapters.

Hello ulf,

Have you ever tried using the adapter with other programs besides VAG-COM?

Hello.
Wolfgang.

Wolfgang, syncro16 wrote:

Have you actually ever tried the adapter with other programs besides VAG-COM?

Hi Wolfgang,

No, I am "limited" to VAGCOM .