The CAN bus connections are critical just like many others such as the power to the RoboRio. Since the CAN bus wires are integral with the Talon SRX, teams who use a crimp to join two of them together will have to cut the wires when re-using the motor controllers, making the wires shorter each time. The wires can be soldered and then un-soldered but soldering has it’s own problems. This is where teams have to learn to do a good job and learn to enforce an adequate level of Quality Control (treat them like one would treat the braking system in ones car).
The CAN Bus wires are much smaller than the wires normally used with the Anderson connectors. They may have to be folded over more than two times for the crimp to hold the wire in properly. If not done right, the wire may pop out with a slight pull later. I am also concerned with the relatively high un-mating force of the Andersons. Many people pull on the wires to un-mate connectors even though that is bad practice. With the 14-10 AWG wire normally used, the wire and crimp have a decent chance of staying together. With the small CAN wires, I can foresee people breaking the wires.
We just started using them at work and got them from Home Depot or Lowes until our regular wholesale suppliers got them in stock for us.
Some fairly expensive equipment or some very specialized 3-D field-solver software would be needed to determine, objectively, the effect of a connector on the CAN Bus impedance. In general, a connector with contacts that have dimensions and physical configuration most similar to the rest of the transmission line (the wires) will probably introduce the least disturbance to the impedance and the least corruption of the signal. Thus, the locking connectors from Hansen, or something like them, are probably the best solution. An added bonus is that the “Latching Male Housings” from Hansen seem to have a shroud so that when male pins are inserted into them, they cannot short to anything unless one pokes something into the end.
The CAN Bus wires that come with the Talon SRX’s are probably larger than the wires in ribbon cable and might not work with IDC DB-type connectors. There are IDC connectors for larger wires but it would take some research to find some that are readily available in the small quantities that FRC teams would want to buy (i.e. no need to order in 1000’s).
It just occurred to me that we may be using some locking IDC connectors with 18AWG wires at work. I will have to have a look around when I get to work today.
We have had some interesting debates about this while trying to define the best practice for controller locations. It probably warrants it’s own thread but I’m too lazy to start one. We currently have defined 8 subsystems here we might use closed loop control with Talon SRXs. The side debate is whether to distribute them close to the motor / feedback location with a longer and more vulnerable CAN bus or place them in a group with a short CAN bus and longer motor and feedback leads. I leaned toward the former but lost the debate and the 8 Talons are in a small group which my be best as a CAN wire problem takes down the whole bus where a feedback sensor wire affects one device. As a side note we have been distributing the CAN bus around our robots every year since 2010 with minimal problems after debug.
To the point of the thread, since the SRXs are close to each other, we pulled out the terminal blocks from past KITS of PARTS (I think they were donated by AutomationDirect) and used them to daisy chain through. It works well for the layout.
As for the feedback sensor wiring, I had already ordered the latching plugs from Hansen Hobbies that Peter mentioned above. I have been using them for months on my 3D printer without problems. I’m passing the hot end temperature thermistor through one that is constantly being jogged at high speed and it has caused no problem. I wouldn’t be afraid to use them on the CAN bus but I use grey wire nuts on small wires all the time too.
The best connectors have a profile similar to the diameter of the wire. The reflections occur at impedance mismatches and the impedance of the connector is largely a function of the geometry. In an ideal world the wire and all the connectors would have the same impedance at 1Mhz and the terminations at each end would be 2X that impedance, nominally 50-60 ohms for CAN. Note that this is impedance, not purely resistance.
We have used PWM connectors and Molex connectors (for 20AWG wire) with zero issues running 9 Talons. Powerpole pins are overkill for the current and are larger than 20AWG wire - I would shy away from using them.
Personally, I’d love to see that block be part of the TalonSRX if a hardware rev gets performed in future years.
For that matter, I really wish the 10-pin “encoder connecter” were also weidmuller connectors – the 5x2 .05" header connector is less than optimal for interfacing to directly.
I agree that using Andersons for CAN lines is definitely overkill. A good, cheap alternative we found is to use JST SM connectors found over at Adafruit. Solid connection, easy to attach (no special crimper required), polarized, and CHEAP! http://www.adafruit.com/products/319 http://www.adafruit.com/products/318
No need to use a jackhammer here, folks! Not everyone is made of money!
We are looking at using the locking PWM connectors from Hansen Hobbies. They use different contact pins than the “normal” PWM connectors they sell but the same crimper can be used. Like the PWM connectors commonly used in FRC, they have a physical configuration that is more similar to the physical configuration than most of the small connectors I have found (and many of those suggested in this thread).
The 30A and 45A Anderson connectors can work for the CAN Bus connections but will probably present a larger disruption to the characteristic impedance than the PWM connectors. They are also meant for minimum wire sizes of 16AWG and 14AWG, respectively, so extra precautions will have to be taken to ensure a good electrical and mechanical connection is made. It is also likely that people pull on the wires to un-mate the connectors. While the 16AWG to 10AWG wire that is normally used might survive this type of abuse, the 22AWG CAN Bus wires that are part of the motor controllers will be more likely to be damaged.
I have used my industry contacts to find a small connector for the CAN Bus connections but have yet to find a better solution than the parts from Hansen. The parts I was able to find either required the purchase of a special tool from the connector manufacturer (typically $100’s) or were rated for an unacceptably low number of mating cycles (i.e. 25 vs 10,000 for the 45 A Anderson and others). It would be best to check the tooling required and the mating cycle rating before committing to a connector type.
We looked at several connector solutions to make potential Talon change outs easy during competition. A lot of the polarized solutions we considered had costs of ~$1 per connector and used the small crimp pins that the kids sometime struggle with. Then we discovered the WAGO 222-412 2-terminal block. We bought 50 of these for about $0.35 each at Newark (link). The WAGO is small, takes a wide range of wire gauges (28-12 AWG), and has nifty little levers for thumb or WAGO tool operation.
We initially used several of the small terminal blocks that come with the annual KOP to wire up the prototype electrical subsystem and had trouble with an unreliable CAN bus. All our CAN communications issues disappeared after swapping in the little WAGOs.
We are using the same connectors that would be on the Talon SR or Victor so if we ever decide to utilize PWM over CAN, we can do so without having to change the controller wiring.
