VAGCOM Adapter: Performance Requirements

Hello, especially to the digital experts.

After evaluating some frustrating experiences (from other users and my own) with optocoupler adapters, I believe that the cause of most communication problems lies in signal delays introduced by the adapter.

Basically, all electronics have such response times because the internal switching processes from high to low and vice versa require some time.

It appears that a particularly error-prone situation arises when the positive and negative signal edges are delayed differently, because this not only delays the bits in the adapter, but also alters their duration.
For example, in my particular adapter 2.2, saturation effects in the optocoupler outputs reduced the duration of bits received from the car by approximately 35 µsec, or completely eliminated any signals with a duration of less than 35 µsec.

Despite these "performance data," communication with the control units for the engine, ABS, and WFS of my truck was stable, but it was hardly possible to establish any communication with the airbag control unit.
The latter has the highest measurement block transmission rate at 2.6 Hz, and therefore places the highest demands on time precision.

Trimmers in the input circuit of the optocouplers (see adapters 2.3 or 2.4) can be used to equalize the rise and fall times of the signal edges, thereby minimizing or even completely eliminating length errors in the transmitted bits.

Communication with my airbag control unit also works perfectly fine via such an adapter, even though the absolute bit delay due to the low slew rate at the output of the optocouplers still amounts to several microseconds.

The small adapter circuits I built without optocouplers exhibit signal edge delays (of the significantly steeper signal flanks) between approximately 0.5 and 3 µsec, and signal duration distortions between about 1 and 2 µsec.

For me, this means that an optocoupler adapter that is meticulously adjusted to the specific PC it's used with can transmit the bits relatively slowly, but completely without errors, and thus theoretically enable higher bandwidths than (simple) transistor circuits, which can transmit the bits much faster, but at the cost of potentially corrupting them.

My question is now: From what size of error, regarding...
a) Delay in signal transmission between the car and the PC, and vice versa.
b) Bit length distortion = runtime difference between positive and negative signal edges.
Can communication problems occur with VAGCOM in the worst-case scenario?

This should help determine to what extent VAGCOM adapters without galvanic isolation between the vehicle's and the PC's circuits have a legitimate reason for existence based on their system characteristics, or whether they "only" offer the advantages of usually easy compatibility with the used PC (i.e., plug-and-play with a quasi-guaranteed functionality without requiring any adjustments), as well as potentially lower material costs, in exchange for their electrically limited operating range and reduced protection against damage to the PC and vehicle electronics.

"As a digital design enthusiast, I would also find some simple rules of thumb regarding achievable baud rates as a function of the time delays of positive and negative signal slopes to be helpful."

For the sake of clarity: My intention is not to criticize Maik's adapter, but rather to have these aspects clarified from a purely technical perspective – also as a helpful resource for those who may consider buying or building an adapter in the future.

Hello Ulf,

When digitizing, it is important to adhere to the Nyquist criterion. This states that, when digitizing signals, a minimum of two samples must be taken within one signal period (the theoretical minimum!). The highest signal frequency is represented by a square wave (a sequence of 0s and 1s). Therefore, the cutoff frequency of the circuit is also significant. It is important to consider the voltage levels at which connected devices can reliably recognize a 0 or a 1.
The definition is specified for the serial interface. I am not familiar with the definition for the engine control unit.
Therefore, the question is: What do we know about the interface components in the vehicle?

AndyO wrote:

Therefore, the cutoff frequency of the circuit is also significant. For this, it is necessary to consider up to what levels the connected devices can reliably recognize a 0 or a 1.

Hi Andy,

If I remember my oscilloscope measurements correctly, I had a slew rate in the order of 1 Volt / µsec at the RXD output of the adapter (2.4). Unfortunately, I didn't write it down .

Quote:

Therefore, the question is: What do we know about the interface components in the vehicle

?

Disclaimer: Values above 60% for Ubatt are considered "High," while values below 40% are considered "Low."

However, I don't think that should be the problem, because the T1 and T2 components behind the optocouplers convert the relatively gentle signal edges into quite sharp rectangular pulses.
You only need to adjust the switching threshold of approximately 1.2 volts using VR1 and VR2 to match the OK output signal, ensuring a consistent delay for both positive and negative signal edges at the OK input.