CAN Bus Layout

[Dale]

A slightly different CAN problem has now emerged. Some of our Jaguars are working and some not. As soon as the CAN daisy chain reaches one of the inoperable Jaguars, all of the rest of the Jaguars downstream from it are invisible.

Are the pins on both of the CAN sockets on the Jaguar wired together in parallel or does the Jaguar process the CAN bus signal in some way and rebroadcast it? Could a bad firmware load or some setting we accidentally made cause this?

A related question would be does it matter, except for RS232, which of the two connectors you plug the CAN cables into? We've been careful to always go into the one with the 10101 on it and out the other but if this is a true bus I suspect it doesn't matter.

[Kevin Sevcik]

The schematic shows all the pins as parallel. If there's an electrical problem with the jaguar that's holding one of the data lines high or low, then that should affect all/most of the jaguars on the bus, not just the ones "downstream" from the bad jaguar. Perhaps your have a jag with a bad connector on it? You should be able to make a pigtail cable for each port and check for continuity between each pin. There should be continuity between pins 2-5 from one connector to the other.

[EricVanWyk]

There is no rebroadcasting, the signals just pass right through.

I'm guessing you have a bad cable going from the last operational jaguar to the first invisible jaguar.

[Dale]

It's more than one Jaguar that's causing the problem. The only thing I can think of is maybe there's a problem with the terminator we made. We'll check that today. That might explain why the first few Jags in the chain work but further out some work and some don't.

We've swapped cables around from known good ones earlier in the chain. That didn't help.

We're just using standard RJ11 connectors. Is that okay?

Any other ideas if that one doesn't pan out?

[dyanoshak]

How many Jaguars do you have on your bus?

How long is the total length of your CAN cables?

Check out Table 8-1 "CAN Wiring Parameters" on page 26 of the MDL-BDC24 Getting Started Guide. It states the total length of cable should be 20 ft or less. Start with 20 ft of cable, and cut all your cables from that length.

Also, make sure that your terminator resistors (in the RS232 housing and at the other end of the chain) are both 100 ohms.

And, no, it doesn't matter, except for RS232, which side you plug a CAN-only cable to.

Hope this helps!

-David

[Dale]

The total length of our bus is only a few feet at most. We're using the 2CAN so it has an internal 120 ohm resister at one end. The other end is the terminator we haven't really tested.

The total system will have 10 Jaguars though for debugging purposes we've shortened that to five.

[dyanoshak]

You replied while I was writing this:

"I just noticed the 2CAN documentation states that there needs to be 120 ohm terminators on the network. This is a standard value for CAN.

However, in the Jaguar documentation, 100 ohm terminators are specified because they help reduce errors on the bus when the node count gets higher."

So I'm pretty sure it will work if you remove the jumper on the 2CAN, and add your own 100 ohm resistor to each end.

Keep me updated,
David

[Alan Anderson]

Originally Posted by Dale

The only thing I can think of is maybe there's a problem with the terminator we made. We'll check that today. That might explain why the first few Jags in the chain work but further out some work and some don't.

Here's a bit of lore that has helped me track down similar problems. Termination issues tend to cause trouble at the other end of the bus from where the fault is. If the "far" devices are where the flakiness is showing up, focus on the "near" terminator.

In this case, the built-in 2CAN resistor might not be the right value for your heavily loaded bus.

[ozrien]

We have ran 6 jaguars with a 2can and one end and 120 ohm resistor on the other with no problems. Our total cable length was also only a few feet.

I would like to know if anyone has had issues solved by replacing the 120 ohm with 100 ohm. ISO11898 specs 120 ohm at each end of the Dual Wire CAN, and I have never seen an automotive CAN network that deviated from this.

[Dale]

We seem to have isolated the problem to a batch of bad RJ11 connectors. It seems the connectors we got (from Fry's) don't reliably mate with the ones in the Jaguars. Is there a particular brand or vendor we should be using?

[ozrien]

We've been using RJ12s from radioshack.

[Dale]

To close the loop on our CAN saga. We have it all working now with 10 black Jaguars. It was quite a haul but it CAN be done

What we found:
The Jaguar's seem more sensitive than we would have thought to the RJ11 connectors used in them. Things didn't get reliable for us until we used the actual AMP/Tyco RJ11 connectors specified by Luminary Micro. There are no doubt others that will work but, for example, the ones from Frys and Home Depot were quite flaky for us. Ours came from Mouser http://mouser.com/Search/Refine.aspx?Keyword=5-641335

We tried making the terminator by just crimping the wires of the 100 ohm resistor into the RJ11 connector. That turned out to be unreliable in our case. Everything is fine now that we're soldering the resistor on the end of a short cable (the same stuff use use for the other CAN cables) coming out of the connector.

We're using the 2CAN Ethernet/CAN adapter distributed by AndyMark. It's nice in that you get loads of diagnostic data, don't have to worry about sending commands too fast, and it downloads firmware to the Jaguars in a couple of seconds rather than forever with the RS-232 interface. The RS-232 interface can probably work fine, too, but we were so excited to finally get something to work with CAN we didn't want to mess with it any more.

Triple check that all of your Jaguars have the new firmware downloaded from http://www.luminarymicro.com/jaguar

[Al Skierkiewicz]

Dale,
You have found by experiment that some types of crimp connectors are not designed to terminate solid wire, the resistors. They are designed for the stranded, flat wires generally available where phone parts are also stocked.