Need help extracting speed data with an esp32

[kamikaze2112]

I'm trying to write an app that does nothing more than extract actual ground speed from the isobus. I'm developing a one-off controller for an air seeder that will be used in conjunction with another implement.

If my understanding of this is correct I need to First initialize a can driver, I want to use the ESP32s built in twai driver.

Then I need to register on the network by providing a name, and then I need to send a PGN request for the PGN I want to extract speed data from and possibly a response frequency.

What I'm confused about is where exactly do I send that request? Do I simply broadcast it and wait for a reply? I don't want to be constantly flooding the bus with requests for Speed, but I would like it updated fairly often.

My plan is to use the Arduino framework, but I can use ESP IDF if necessary. I will admit that I'm finding this Library a touch overwhelming, and I've possibly bitten off more than I can chew.

I did manage to get one of the virtual terminal examples to compile. It seemed to be receiving data on the bus but I'm not sure how to pick out what I need in that example.

Any help would be greatly appreciated.

[Matthieu-Poulain]

Hi there, i'm not completely sure so this may be wrong. I'll have to do the same probably soon anyway so we might as well try it.

As i was building on the AgIsoStack (with IDF) and following the tutorials, I took note of the following:

                /// //////////////////////////////////////////////// ///
		///     Additionnal callbacks  registreings          ///
		/// //////////////////////////////////////////////// ///
		{
			/** Note:
			 * isobus::CANNetworkManager::CANNetwork.add_global_parameter_group_number_callback(0xEF00, (isobus::CANLibCallback)test_a_callback, nullptr);
			 *
			 * The nullptr variable is a generic context variable. Whatever you pass in for that variable will be passed into
			 * broadcast_test_a_callback when it is called later. This can be useful for figuring out what object wanted the callback.
			 * For example, if we were registering this callback for a particular class, that class could pass in this as that
			 * argument to have its own pointer passed back to it in the callback.
			 **/

			// Broadcast  messages processing                                                   PGN,              Callback                      Parent
			// isobus::CANNetworkManager::CANNetwork.add_global_parameter_group_number_callback(0xEF00, (isobus::CANLibCallback)test_a_callback, nullptr);
		}

		{
			// Receive of specific messages
			//                                                PGN,                  Callback,                Parent
			// myVtPartner->add_parameter_group_number_callback(0xEF00, (isobus::CANLibCallback)test_a_callback, nullptr);
		}

As i understand it, this is a way to link a callback to the PGN message you want to receive. When received, it will trigger the callback and you will then only have to process its data inside the callback ...
What i do not know however is if the message that contains the speed is a broadcasted message, even when no one asked for it to be sent or if you have to trigger the message by yourself.

If this message is not broadcasted you might have to request for it.
My guess would then be to have a second thread that sends the request every now and then, with a timer triggering the sending. The main question is how often do you need the speed to be refreshed. If its every second it should not be that terrible, while if it is every ms, that may be worse.

[Matthieu-Poulain]

Hi there,
Not sure if you are done with the subject but i'm field testing things next week to get the PTO enable value; If things go well that would mean you should be able to do the same to get the data from the J1939 frames that are periodically sent even without any request from you. The data you are looking for may be found in the PGN frame 0xFE48

Bytes1, 2: theorical speed