Hi,
I am attempting to hook my Under the Hood model to the OBD2 port to get SavvyCAN/M2RET working on it to do some packet sniffing. However, I ran across the issue is that on the UTH model there are 9 pins listed as “Digital Ground” on the 24 pin connector. I don’t which one of those to connect to pin 5 on the OBD2 connector. I bought an OBD2 pigtail to connect the UTH to the OBD port to make things easier and more reliable (I hope).
Thanks,
Douglas
When you see a lot of pins labeled “digital ground” what that means is that they’re all equivalent. That’s pretty common on devices like this. What the circuit designer usually does (at least what I do) is lay out all the signals they really need and then they take every pin left over and just assign kinda-sorta useful things to those pins. Usually that means extra ground and power pins. Ground pins are a common “filler” because they’re useful. You need a digital ground for power to the M2, and for single wire CAN, and probably for some of the other buses. So, having lots of digital ground pins gives you a way to hook up all those things with their own ground connection.
Perhaps that was an overly long explanation. The short answer: use whichever one you want. They’re all the same.
Collin, thank you! I appreciate the explanation to go along with the answer.
So that brings up another question, you mention that it needs a digital ground for power to the M2, however pin 12 on the M2 is listed as a Chassis ground. I would assume that is the negative side of the power, not a digital ground pin. Also, am I wrong that CAN requires a ground or does it just need high and low? It makes sense that the SW CAN needs a ground.
This is the site I was basing the assumption that I need the digital ground: https://www.kanda.com/blog/microcontrollers/bus/
“CANUSB could be used to read CAN signals from a vehicle by connecting CAN_L, CAN_H and CAN GND to the correct pins on OBD-II connector”
Chassis ground is not the other side of the incoming power. It tends to be connected to digital ground on most boards but through a resistor and capacitor (RC filter). The chassis ground is really used for things that have a shield. You connect the shield wire to chassis ground and the actual signals use digital ground.
Normal CAN does not need a ground connection. CAN_L and CAN_H are differential and only need to reference each other. You are allowed to connect a common ground between nodes on the CAN bus. If you do that and one of the L or H lines breaks then the transceiver might still be able to work by referencing the other line to ground. But, you get less isolation between nodes if you do this.
Ug, so many terms I need to learn and they aren’t translating well from my automotive knowledge. Thank you so much.
To clarify, if I connect OBD pin 4 and 5 to chassis ground and nothing to the digital grounds, will the M2 work correctly?
Or should it they both go to a digital ground?
Or pin 4 to chassis ground and pin 5 to a digital ground?
I am communicating with K-Line and CAN.
Pin 5 of OBDII is the digital ground. Pin 4 is the chassis ground. You seem to realize this. So, I’m just clarifying.
Now, the question is, will it work if you hook both to the chassis ground of the car? Well, yes, probably. In a car the entire chassis is usually connected directly to the 12V negative lead. So, connecting to chassis ground is essentially equivalent to connecting to the ground of the car. You could also connect both to a digital ground wire. That would work too. The correct approach is to connect OBDII pin 4 to the chassis ground and pin 5 to a digital ground.
Now, the deeper question is, what is the difference? The car chassis is ground, digital ground wires must also eventually make their way to the 12v negative terminal somehow. But, there is a difference. Dedicated wires can potentially pick up less “chatter” as they haven’t got as many things connected to them. The chassis in a car is quite noisy. In an internal combustion engine car you’ve got Tesla coils connected to spark plugs. They’re noisy. In an electric car you’ve got potentially 1000A of current getting switched on/off 10,000 times per second. Either way, the chassis of the car could be a noisy place. Dedicated wiring can be insulated from this a little bit better. In a general sense, from a high overview, the chassis and dedicated digital wiring is equivalent. From a closer, more technical perspective they’re a little bit different. If you can help it it certainly doesn’t hurt to do it properly.
But, you’ll probably be OK with any of the three options - connect both to chassis, connect both to digital ground, connect them separately.
Thank for the clarification. I’ll add a digital ground to pin 5. Maybe the cleaner signal will help with my issues connecting properly.