Comma AI self driver assistant

[Ajzride]

Also, the CAN-BUS specification calls for 120-ohm termination at each end-point. I don't know your schematic - are these being connected in serial? 62 ohms I think would load the bus too much. Apologies if you're figured all this out (I see you're an engineer also). Feel free to tell me to mind my own business

Sixty-two ohms was arrived at by trial and error. I believe I started with 120. Been a while now, I can't remember exactly.

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[UncleTony]

Too easy to bump a cable loose. I'm working on one today that is soldered and sealed for someone who can't wait for the PCBs to be developed.

Ideally, a breakout board without the headers would be a better solution, but I was trying to make something that someone that doesn't know how to solder very well can make on their own while the permanent solution is made. I wouldn't be super worried about securing these any better than they already are. Now that they are heat shrinked, the only concern would be accidentally having them unplugged when you're cleaning your windshield or something.

The thought was to 3D print a project box for it, but it would be too bulky with the headers.

This also allows you to keep spare resistors soldered up and stashed in the truck in case something goes wrong.

 

[bmwhitetx]

It's an electronics issue having to do with canbus signal reflections on an unterminated stub. You can read the whole techincal discussion here:

https://github.com/commaai/openpilot/issues/30302#issuecomment-2681736903

The summary is we need to add resistors close to the rearview mirror to kill off the reflections. Currently that is done by extending the harness and using only the short USB. But in theory you could use a long USB and a short USB (like they sent you) with some resistors in the coupler. That is what I'm going to try and make this week.

I read through most of the discussion in the link provided above. CANBUS needs 120 ohm termination at the start and end of each circuit and for devices at the end of long cable runs. The reason the one guy measured 60 ohms is that the circuit had a 120 ohm termination at the Gateway and another 120 ohm at the APIM. Those resistors are in parallel to each other so one would measure about 60 ohms. If you add another 60 ohms in parallel you are loading the bus more than intended. I get that "it works" but I don't believe it is ideal.

Typically with short stubs, no further termination resistor is needed to reduce reflections. But sounds like the Lightning requires a long connection cable.

I really think you should change your resistors out from 62 to 120. Could probably still work at 150 or 180.

EDIT: If you already bought a pack of 60 ohms, just put 2 in series (that makes 120) and test.

 

[Ajzride]

I read through most of the discussion in the link provided above. CANBUS needs 120 ohm termination at the start and end of each circuit and for devices at the end of long cable runs. The reason the one guy measured 60 ohms is that the circuit had a 120 ohm termination at the Gateway and another 120 ohm at the APIM. Those resistors are in parallel to each other so one would measure about 60 ohms. If you add another 60 ohms in parallel you are loading the bus more than intended. I get that "it works" but I don't believe it is ideal.

Typically with short stubs, no further termination resistor is needed to reduce reflections. But sounds like the Lightning requires a long connection cable.

I really think you should change your resistors out from 62 to 120. Could probably still work at 150 or 180.

EDIT: If you already bought a pack of 60 ohms, just put 2 in series (that makes 120) and test.

Please forgive the incomprehension of this chemical engineer, but how does 60 ohms “load up” less than 120 ohms. doesn't more resistance create more load?

 

[bmwhitetx]

Please forgive the incomprehension of this chemical engineer, but how does 60 ohms “load up” less than 120 ohms. doesn't more resistance create more load?

Yeah, sorry if I stated that too vague. The bus is already at 60. Adding another 60 in parallel makes it 30. You have lowered the resistance. That loads the bus (more current flows). At some point you have a short circuit.

Edit: I = V/ R The less resistance, the higher the current (load).

 

[Ajzride]

@UncleTony can you try 120ohm? I’ve already epoxied in the 62ohm on the one I am shipping out.

 

[UncleTony]

Yeah, sorry if I stated that too vague. The bus is already at 60. Adding another 60 in parallel makes it 30. You have lowered the resistance. That loads the bus (more current flows). At some point you have a short circuit.

Edit: I = V/ R The less resistance, the higher the current (load).

Wouldn't that depend on if it's canbus or a usb controller handling the USB C cable?

62 ohm is correct in terms of USB C

 

[Ajzride]

Wouldn't that depend on if it's canbus or a usb controller handling the USB C cable?

62 ohm is correct in terms of USB C

It’s not USB at all, it’s canbus. They just use a usb cable.

 

[UncleTony]

It’s not USB at all, it’s canbus. They just use a usb cable.

It seems weird that the head unit wouldn't be using a usb controller to run the connection between the unit and the harness, but I can give it a shot.

@bmwhitetx - you think 2 62 ohms would work?

 

[Ajzride]

It seems weird that the head unit wouldn't be using a usb controller to run the connection between the unit and the harness, but I can give it a shot.

@bmwhitetx - you think 2 62 ohms would work?

They call it ODB-C. It's just a convienent 24 wire cable for them:

https://github.com/commaai/neo/blob/master/car_harness/OBD-C.sch.pdf

 

[bmwhitetx]

It seems weird that the head unit wouldn't be using a usb controller to run the connection between the unit and the harness, but I can give it a shot.

@bmwhitetx - you think 2 62 ohms would work?

Yes, I would try (2) 62 ohm resistors in series. Doesn't have to be exactly 120. It's about putting a reasonable load on the bus.

And yeah, from what little I have read, they are using a USB-C for convenience. Not using the protocol at all.

 

[UncleTony]

Yes, I would try (2) 62 ohm resistors in series. Doesn't have to be exactly 120. It's about putting a reasonable load on the bus.

And yeah, from what little I have read, they are using a USB-C for convenience. Not using the protocol at all.

I guess i knew that. I've been stuck in USB land between mutilating the cable and making the breakout board.

It works the same using the 2 62 ohms in series.

I guess I'll make new sets with .25w 120 ohms.

Im starting to miss radio shack at this rate lol.

 

[UncleTony]

I ordered these https://a.co/d/dwXYm1v. I will have them on Thursday and report back once they are installed.

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