AVR Freaks

RS485 is an electrical standard. CAN is both the electrical standard and the protocol.
If you reduce the question to "This 8 pin transceiver vs. this other 8 pin transceiver", the difference becomes pretty simple:

An RS485 transceiver has a threshold close to 0V differential, and drives +3V/-3V or so into the cable. The noise margin is roughly 3V.

The CAN transceiver is half an RS485 transceiver with receiver offset. It drives +3V/HiZ, and depends on the termination resistors to bring it back to 0. In order to make signalling work, the receiver differential threshold is around 1V, and thus the noise margin is around 1V.

My suggestion is to go with RS485. CAN transceivers are the way they are because they need to perform wired-AND on the bus, which is required for CAN arbitration to work.

I've done RS485 over 1500m at 230kbps. And it looked pretty on the far end. So 40m is nothing.

/Kasper

If RS232 is chugging along .. why bother.

It was originally designed for longer length than 40 metre.

RS-422 if you need noise protection, RS-232 otherwise. RS-485 and CAN are both unnecessarily complicated for this application. Between those two - I'd probably go with RS-485 as you should be able to get up and running faster (CAN takes a little while to understand). You will have to design your own protocol for RS-485 though, which could be a pain.

I'd use RS484. It's simpler to change RS232.

As for the wires. Don't you scare the lightnings? If it strikes your cable you'll have ashes instead of your notebook)))

This noise margin only applies to differential signal levels in the CAN recessive signal state. The common mode noise rejection is obviously far higher. Each CAN node has its own passive bus- pull down, so the primary job of a simple termination design is to terminate reflected signal energy at the bus ends, although it does also provide a DC/AC load for the CAN bus. There are also designs with weak termination on each stub node. Actual real world CAN bus performance is impressive since it provides reliable up to 1 mbps communication in environments like an automobile electrical system.If you get a direct hit, no protection of any kind will work or make any difference. The wires will turn into metallic vapor. Lightening problems are usually not so much from direct hits, but from near misses. The huge ground currents and induced energy from a near miss will still turn solid metals into charred debris, let all the smoke out of your components or do something as delicate as unsoldering connections :wink:. The not too near lightning strike miss is where circuit/wiring lightning protection comes in handy. Your 30 meter tower is an invitation for a near miss on your 3 meter tower.

I question if you really had a direct hit on your tower. One of our local guys dealt with a properly grounded metal container building converted into a radio vault that took a direct strong hit. There were several holes in the roof that you could put your fist through and radio racks were blown all the way across the vault, then a fire inside the building burned the equipment. Also keep in mind ground rods grounding quality often deteriorates over time, depending on the soil type. We recently dealt with a communications antenna that took a near miss. It still worked, except the wind induced Rx noise in the antenna, but the hard line survived. The repair guy found what "looked like" carbon granules in the base of the antenna. This antenna is designed to absorb as much damage as possible in its base to help save other equipment, if possible.