Help in deciding between RS485 or CAN [closed]

Question

PROBLEM

I'm building a robot control board to attach to a Raspberry Pi as an hobby. I like to have specialized boards for various functions, and I'm exploring solutions for the communication bus. In the past I tried:

• 5V UART: problems with noise
• I2C: just doesn't work with noise
• POF Plastic Optic Fibers: Cool but expensive and large
• RS485 over audio cables with 3.5mm jack connectors with custom protocol: Works like a charm. I like how inexpensive the connectors and the cables are.

Below an image for reference of one of the robot I made in the past using a RS485 bus with custom protocol over audio cables with 3.5mm jack.

Below an image for reference of my latest iteration of the Raspberry Pi robot hat. I want to add display and connectors for a communication bus, add a better power regulator and move motor and servo controls over to slave boards.

IDEAS

I'm thinking of either implementing a CAN bus or a RS485 bus with a less custom protocol that has some use in the market.
Here an example of a slave that uses CAN bus
Here an example of a slave that uses RS485 bus with their custom protocols.

Specifications:

• Three wires (two data + one shield), so I can use audio cables and 3.5mm jacks for my slaves
• I want the bus to be less custom, so that I can attach it to existing add on boards
• 1Mb/s bandwidth
• Resilient to noise

QUESTION

I'd like suggestions on which bus you would use in this application, in particular if there is a bus that I have not taken into consideration, suggestion on a standard protocol over the RS485 bus or your reasons you would choose a CAN bus. Thanks for your inputs!

SOLUTION

There are no dominant protocols used with RS485 and CAN has a large data frame overhead.
I decided to implement two 3 pins 3.5mm Jack connectors with a RS485 transceiver connected to an AT4809 for the hardware. Over the RS485 bus I will reuse my custom master-slave protocol.

Below an schematics and initial placement of components.

Answer 1

I'm afraid you are comparing apples to oranges.

RS-485 is simply a way of driving a bus. You take your UART output, translate it via an RS-485 transmitter, receive it with a receiver, and then feed that into the destination UART. Easy peasy.

CANBus is a much more complicated proposition. You have to feed a CAN controller with data packets. The controller then drives the physical bus with a far more complicated protocol to the receiver, and the receiver produces a reconstructed packet. In effect, the two controllers would replace your UARTs.

As for reliability, CANBus is better. All the extra garbage that a CANBus uses go toward two things: selecting one out of a number of units which are all connected to the bus, and error detection and correction.

When you say you want 1 Mb/s bandwidth, you will never get that from CANBus. It's true that you can drive CANBus at 1 MHz, but if you send 1 byte at a time you'll only get an effect 140 kb/s. It takes 58 clock cycles to send a 1-byte CANBus frame. It's true that it's possible to drive the bus at 5 MHz, but that only gets you up to an effective 700 kb/s.

Although, to be fair, a UART driven at 1 MHz will only have a throughput of a 700 - 800 kb/s, depending on your choice of frame setup, but you can readily drive it faster than that.

See https://en.wikipedia.org/wiki/CAN_bus for an overview.

It's also normal for a single CANBus source to provide the 5 volt power for all the other units. This means you need 4 signals on your bus, and an audio wire won't cut it.