Using a star configuation for Jaguars on CAN

[Al Skierkiewicz]

Dale,
The six pin connectors are the only way to go here. The RJ-11's will move slightly in the connector body of the Jaguar and lift one or more of the spring wire contacts. If you examine the connector you will see a slight rise of plastic between each contact on the plug. If the wire doesn't sit down between the two adjacent plastic bumps, the wires move and either touch each other or don't contact the receptacle wires. I don't know why the sheet and FAQ recommend the 4 wire connectors other than those are easily available parts. In most cases if the plug is inserted properly, there is no problem. With a moving robot, and insertions in a confined space, the chance of accurately inserting the plug is vastly reduced.

The terminators in this kind of arrangement add some noise immunity while maintaining the transmission line characteristics of the daisy chain in both directions. If a star configuration were used, it would need to use active drivers each feeding a separate leg of the star. A simple star arrangement with a terminator on each leg without the active drivers would pull the buss impedance so low that the individual drivers would not be able to supply enough current to communicate via the buss. If you check the Jaguar schematic you will find the CAN sense input which is simply a voltage divider feeding the controller. If the voltage at this point falls to a level too low to communicate the controller will simply communicate with the CAN buss.

Just to remind everyone, UTP stands for Unshielded, Twisted Pair and that a four wire UTP has two twisted pairs, a six wire has three twisted pair and a four wire, Ethernet, has four twisted pairs. Each pair acts as a balanced transmission line and in the case of CAT 5 and 6 cables the pairs are twisted at different rates (twists per inch).

[Dale]

Al,
Thanks for the input. We'll definitely try the six pin connectors in the fall.

I wasn't thinking of terminating each leg of the daisy chain since, as you mention, that would lower the impedance too far. I was thinking of just a 100 ohm resister at the middle of the star (or maybe 50 ohm with nothing on the leg coming from the 2CAN.) It sure isn't ideal from a noise standpoint, though. The active driver would be terrific but beyond what we'd want to take on.

We never considered using UTP (twisted) cable because it's such a pain to use. Untwisting all those pairs and getting them in the right order seems like an error prone process. All of our cable was straight through like they picture in the Jaguar Manual in appendix A. Do you think UTP is necessary?

By the way, we found our terminators were much more reliable when we soldered the resistor on the end of a short pigtail crimped into the connector. That's probably because our RJ11s were designed for stranded wire.

[Hugh Meyer]

Dale,
We used Ethernet UTP cable with the six pin connector. We found it best to only connect the two center pins, which are the CAN_H and CAN_L signals. I would suggest NOT connecting the pin labeled ground. The ground signal just creates ground loops and is not needed. After we disconnected the ground circuit our system became much more stable. We had 8 Jaguars on the bus. Our final cable configuration was just two conductors, one twisted pair. We would remove the other unused wires from the cable assembly by pulling them out. I always had the students test the cables with an ohm meter and in a working loop before declaring them done. We put shrink tubing on the resistor leads to prevent them from shorting to each other when we assembled the terminator.
Hugh

[Dale]

Now that's a novel idea. I hadn't thought about just using the CAN_H and CAN_L pins. You make a good point about ground loops. Has anyone else tried this?

[Al Skierkiewicz]

Dale,
In the relatively short connection length that a FRC robot would use, the flat telco wire should pose no real problem. (1/4 wavelength at 1 MHz is about 234 ft.)
I included the reference of UTP to prevent any misunderstanding with the term. The device used in the Jaguar is a balanced transceiver chip capable of driving a 60 ohm load. The Jaguar chip is specified as having 30 kohm input and the Jaguar itself has 20 Kohm from CANL to ground so it is unlikely that 10 Jaguars would significantly load the buss. As I was checking the spec sheet for the Jaguar, I found the following...
"Use RJ11/RJ14 modular cables to daisy-chain CAN communications to any other MDL-BDC devices. The cables should be 6-position with either 4 or 6 contacts installed. Suitable cables have plugs crimped on opposite sides of the cable and are referred to as reverse or straight cables, because pin 1 connects to pin1."
And...
"When controlling more than one MDL-BDC, modular cables (6P-4C or 6P-6C) should be used to link the modules. Suitable cables include the Digikey H2642R-07-ND cable."
I can go along with the ground suggestion, theoretically it is not needed for the transmission line. It does tie each internal ground through a separate connection in addition to the power supply common. This produces ground loops with motor noise and common mode noise added to the chassis common in each device.

[Dale]

For what it's worth, our RJ11s have have four pins installed and six channels, just like a normal teleco connector.

Hugh,
How did you get a good crimp on the cable with the round(ish) Ethernet cable? It would seem like with most of the wires removes the top part of the connector might not grab the outside shell of the cable very well. Did they tend to come off, did you do something special, or did it just happen to work?

Dale

[Dave Flowerday]

Originally Posted by Al Skierkiewicz

Dale,
The six pin connectors are the only way to go here. The RJ-11's will move slightly in the connector body of the Jaguar and lift one or more of the spring wire contacts. If you examine the connector you will see a slight rise of plastic between each contact on the plug. If the wire doesn't sit down between the two adjacent plastic bumps, the wires move and either touch each other or don't contact the receptacle wires.

Al, are you sure you're not thinking of the handset plug (sometimes incorrectly called an RJ9)? An RJ11/12/14* jack has all of the necessary "slots" for the spring wire contacts on the Jaguar side and should not allow those spring contacts to move and touch each other. The only difference between the RJ11, RJ12/RJ25, and RJ14 is that the RJ11 has only 2 positions populated with contacts, the 14 has 4 loaded, and the 12/25 has all 6 loaded (but all 6 positions still exist in each case).

If anything, I would expect that an RJ14 would allow the outside spring contacts to sit even deeper in the plastic "slot" than an RJ12 because without a contact in the outside positions of the plug, the "slot" is actually deeper than a populated one.

* Note that a real RJ11 only has 2 contacts, which seems to contradict the Jaguar manual specifying at least 4 contacts. Seems like they should be recommending RJ12,14, or 25 only.

[Al Skierkiewicz]

Dave,
There is a lot of confusion out there on these jacks including the Jaguar docs. The cheaper RJ11 jacks do not always have the depressions you speak of where the two outside contacts of a six position plug would normally be located. However, I have seen teams using the handset plug (which is narrower than a standard RJ11) this year. Specifying the six position connector prevents any ambiguity. Although one pin of the connector is labeled +5volts, that is not connected to any power supply internal to the Jaguar. If other sensors are used to that provide power on that pin, misalignment could cause some other serious issues. Also a crimper that is designed for four position plugs will not push in on the outer contacts of a six position plugs when assembling these plugs.

[Hugh Meyer]

Originally Posted by Dale

Hugh,
How did you get a good crimp on the cable with the round(ish) Ethernet cable? It would seem like with most of the wires removes the top part of the connector might not grab the outside shell of the cable very well. Did they tend to come off, did you do something special, or did it just happen to work?

Dale

Dale,
It clamps down about like on a RJ45 network connector. We just try to push the outer insulation into the connector before making the crimp. Even if it comes out the wires are still securely held by the crimp pins deeper in the connectors.
Hugh

[DonRotolo]

Originally Posted by Dale

An active star module that allowed every jaguar to remain at it's default address of 0 would be very cool.

That would kind of defeat the advantages of CAN networking though (the added complexity of the "router")

But there is no reason CAN can't be used in a star topology. We use a star topology in the cars where I work, on both the 125 kb/s and 500 kb/s networks. The key is proper terminations.

Terminating resistors are intended to prevent the signal reflections that occur at the end of an open-circuited wire. What happens is the signal hits the open wire end (at the speed of light) and bounces back into the network (at full amplitude). Do this just wrong and your data bits turn into garbage.

A good tool to see if your star network is well-terminated is an oscilloscope. Compare a known-good signal to your network.

Regarding noise: A CAN-Bus is inherently resistant to noise, since it uses differential signaling. It is the difference between CAN-H and CAN-L that determines logic 0 or 1. In theory, both wires of the network will be affected equally by some induced noise, while the difference between the wires remains constant. In practice, the noise induced into each wire is "same" enough to work against noise very well.