Function of the L-line?

Hello everyone.

Based on my experiences from various experiments involving VAGCOM opto-adapters, I find it puzzling why Jeff Noxon directly connects the L-line to an optocoupler in his minimal adapter.

Depending on the optocoupler type, a 10 kΩ pull-up resistor might only allow for a slew rate of a very small amount to almost no volts per microsecond, and with a saturated optocoupler output (depending on the current sourcing capability of the COM port), signal delays of several microseconds could occur.

If the adapter still functions with vehicles that use the L-line (provided it is electrically compatible with the PC to some extent), this suggests that the L-line is significantly less demanding in terms of timing accuracy than the K-line.

This would mean that with my "Opto-Adapters," the transistor output stage for the L-line might be able to be omitted without compromising reliability.

"Does anyone know anything about the meaning/function and minimum requirements for signal quality of the L-line?"

You don't need the L-line with the newer engine control units.
(Regarding triggering), more is not needed because the K-line, +12V, and ground connections are already sufficient.
Best regards, Olli.

Hi, Ulf,
I also think that the L-line isn't really necessary.
I have a part from eBay (because I couldn't manage to solder a working adapter back then ). In any case, the eBay part also doesn't have an L-line.
I can access the engine, ABS, and airbag control units in my 1994 Passat 1Z. However, for example, the central locking system doesn't work. I'm not sure if it even has a control unit...
So far, I haven't experienced any errors, disconnections, or anything like that. However, unfortunately, I can't tell you what's inside because the 'manufacturer' filled the casing with hot glue .

Best regards.
David.