A question (and idea) about MOTOTRBO and CAN bus

The below question came in the other day via via Twitter.

@cwh050 since I could not find someone else I thought you might be able to help: Some cars offer integration of accessories (taxi, police and others) to control some stuff via the onboard systems like mentioned here: https://t.co/t8wmFZszVK (1/2)

— nico (@nicoduck) June 9, 2021

I have some knowledge (although not in-depth) on this topic since I had some ideas a while back. 

CAN bus is a standard that allows devices in a modern car to communicate with each other. It uses a message-based protocol defined in ISO 11898. The higher level protocols are developed and maintained by the CIA (not the US central Intelligence Agency). 

In principle, it is possible to interface a MOTOTRBO radio into a vehicle's CAN bus system and address it using CIA447. With this, one could get radio display information shown on the vehicle's instrument panel or entertainment system. 

The radio cannot be connected directly to the CAN bus, so there would need to be some hardware between the CAN bus gateway and radio to handle the conversion between XCMP (which the radio uses) and CAN.

The second, more challenging, hurdle is that each manufacturer has some minor differences in the instrument panel formatting. This means that in order for such a "magic box" to be used on more than one model of car, it would need a database of vehicle models with some details about each. There would also have to be a user interface that would allow an installer to configure the unit to suit each car.

With a little more effort, you could include the capability to change channels and potentially control PTT from the steering wheel. A word of warning though: the audio quality on DMR is not that great when the user is far from the microphone and in a (noisy) car.

This means that the "magic box" needs to contain a Single Board Computer like the Raspbery Pi maybe. The computer will need to be robust as the temperature in a parked car could reach 70°C in summer and -70°C in the winter - depending on where it needs to travel - as well as vibration.

This magic box will probably need to be UNECE R10 certified if it would be sold in any country that requires this. There may be other regulatory approvals needed.

In order to develop an interface between a MOTOTRBO radio and a vehicles' CAN, companies can become a an Application Developer, more information here. There is no cost to join and you gain access to support from the engineering team.