Oszilloskop-Screenshots der CAN-Signale o.k. ? | Posts 16+

- what exactly?
Baseline? Or is there already a signal present?

Sorry, some information was missing.

Structure:
- without CANUSB adapter and without PC.
- Only a 9-pin Sub-D connector is attached to the connector.
(for later connection of the CANUSB adapter)
- two 120-ohm resistors were soldered into the coupling.
- The CANUSB adapter is not yet connected, nor is a computer.

Measurement 1:
- I'm measuring a CANH signal with a low idle level on the CANH pin of the connector - okay.
- However, I'm also measuring a signal with a low resting level on the CANL pin of the connector, which is not okay.
(Attachment: 'Measurement using an ohmmeter without an adapter')

Measurement 2:
'Only when I disconnect the CANH line does the CANL signal reach the connector without interference.'
(Attachment: 'CANH measurement with disconnected power supply')


How do I get the CANH signal away from the CANL pin?
Do I need to solder in a capacitor there, or should I connect those two 120-ohm resistors to ground?

Best regards,
Volker

Okay, it seems like things are getting mixed up here.

Quote:

- Two 120-ohm resistors were soldered into the coupling.

Theoretically, a 120-ohm terminator should be connected to each end of a CAN bus.
In some cars, tree wiring configurations are used, where there aren't just two ends but more. In these cases, either a 60-ohm resistor is placed in the middle ("central termination resistor") or a resistor >>120 ohms is used in each individual control unit. However, in any case, the CAN bus in the vehicle should already be correctly terminated – meaning that if you connect your adapter to the vehicle's CAN bus, you shouldn't add any additional terminators anywhere, as they are already built into the car!
You may need to reschedule if you detach the bus from the vehicle.

Quote:

- The CANUSB adapter is not yet connected, nor is a computer. Measurement 1: - I'm measuring a CANH signal with a low idle level on the CANH pin of the connector - okay. - However, I'm also measuring a signal with a low resting level on the CANL pin of the connector, which is not okay. (Attachment: "Measurement using an ohmmeter without an adapter") Measurement 2: "Only when I disconnect the CANH line does the CANL signal reach the connector without interference." (Attachment: "CANH measurement with disconnected power supply") How do I get the CANH signal away from the CANL pin?

Most likely, there's a problem with the pin configuration on the connector inside the vehicle.
What kind of connector is that? Are you disconnecting it from a control unit, or is it free?

Quote:

Do I need to solder in another capacitor there, or connect the two 120-ohm resistors to GND?

Absolutely not. First, remove your objections.

Best regards,
Ingo

Thanks for your advice, Ingo – I was afraid I'd been written off as a hopeless case here.

Definitely no. First, remove your resistors.

Yes, I've now managed to adjust those settings, and suddenly the signals are properly high and low at the connection pins of the CANUSB adapter.
Even on the RX of the bus driver 82C251, I'm now measuring a very promising signal...
(attachment: RX without resistors)

... only the can tool monitor remains silent.

When I plug the CANUSB adapter into the laptop, I can read 739 individual packets through the USB port – that's all it does for now.
(Attachment: USB Monitor - CANUSB - Signal Pattern)

If I then connect the can tool monitor, it will send...
The CANUSB adapter then forwards the pin assignment signal to the USB port.
2. A message is written to the USB drive.
(Attachment: 'USB Monitor Connect to CAN Hackermonitor')

It looks like the CAN signals are entering the adapter, but nothing is coming out.

What else can I do right now to get this working quickly? Can I trace the signal further within the 82C251, or what would be the next logical step?

Best regards,
Volker.

Quote:

Thank you for your tip, Ingo - I was already afraid that I would be written off as a hopeless case here... Quote: Definitely no. First, remove your resistors. Yes, I've now managed to adjust those settings, and suddenly the signals are properly high and low at the connector pins of the CANUSB adapter. Even on the RX of the bus driver 82C251, I'm now measuring a very promising signal... (attachment RX_without_resistors)

Okay, that's already a step forward. It's likely that your CAN bus has been operating in single-wire mode so far due to the additional 60 Ohm load.

Quote:

... only the can tool monitor remains silent. When I connect the CANUSB adapter to the laptop, I can read 739 individual packets through the USB port – that's all it does for now. (Attachment: USB Monitor - CANUSB - Signal Pattern) If I then connect the can tool monitor, it will send... The CANUSB adapter then forwards the pin assignment signal to the USB port. 2. A message is written to the USB drive. (Attachment: "USB Monitor Connect to CAN Hackermonitor") It looks like the CAN signals are coming into the adapter, but nothing is going out.

Unfortunately, I can't help you with that right now because I'm not familiar with the Lawicel adapter.

Quote:

What else can I do right now to get this thing working quickly? Can I trace the signal further within the 82C251, or what would be the next logical action?

The Rx signal from the 82C251 needs to be connected to the SJA1000 (CAN controller), which is made by NXP. After that, things get complicated because I'm also not sure how the SJA1000 is connected to the AVR microcontroller.
Ideally, you would have a working combination (car - CAN adapter - software) and then be able to replace components step by step.

Best regards,
Ingo