I'm trying to figure out how to lock and unlock the doors of a 2016 Mercedes Citan using the CAN bus via the OBD-II port. Is there some kind of OBD-II interface where I can control this with plain negative pulses?
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The OBD-II port supports a number of electrical protocols, but all of them are serial in nature. There is nothing you can do to any of the OBD-II pins with "plain negative pulses" to do what you want to do.
One path to what you want is a cheap ELM327 device sold by just about everyone on Amazon.com. They are very inexpensive and easy to use. If you get one that has a USB cable on it, you can talk to it with Putty, or any serial data application you like. Putty is free, and easy to use.
Once you have connected your PC / laptop to the ELM327, you can send commands to the vehicles ECM (Engine Control Module) and anything else that's connected directly to that bus.
Here's the hard part, the command you want is likely going to be a mode 8 PID. (See this link for general information about modes and PIDs Wikipedia OBD link
The reason that's the hard part is that most manufactures don't like to publish what their mode 8 PIDs are, or what the data format is. Getting this information could be difficult, or expensive.
But at the end of the day, you cannot just send single ended pulses to a pin on the OBD port and control anything. Most of the signaling protocols are double ended (differential) btw.
ELM327 Data Sheet
If you read Voyo FAQ it states they use “wireless relays” to add Convenient functions that when an “authorized” device is in range can trigger relays.
So Voyo doesnt appear to be using the OBD port to trigger locks. OBD in the lock scenario is just a coordinator for the Voyo wireless addons relays.