Quote:
Originally Posted by Mike Copioli
David,
I have to disagree with you on this. According to CAN spec IS0 11898:1993(E) Table 15, the maximum cable distance for 1Mbit/second is 40 Meters not 20 feet.
We have also tested at distances much greater than 20 feet without issue using 120 Ohm's of termination on each end of the BUS.
FYI a termination resistor of 100 ohms is outside of the spec. Teams using 120 ohms resistors should have no issues related to signal reflection.
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Yes, that is what the CAN spec says. However, I am talking about what we specify for Jaguar.
They differ in the following ways:
- CAN Spec states 40 meter cable lengths using twisted pair cable
- Jaguar uses non-twisted modular cable
- CAN Spec states 120 ohm terminators
- TI specifies 100 ohm terminators for the following reasons:
The concern is with bit timing, not signal reflections.
Ideally each Jaguar would be configured with its own specific bit-timing parameters (parameters that define when the CAN module samples the bit). These parameters depend on a lot of factors including location on the bus, bus impedance, the number of drivers on the bus, etc.
In this specific application (FRC) a lot of those factors are unknown. What works for one setup may not work as well for another. During our testing of Jaguar we found that as the number of Jaguars increased and the cable lengths got longer, some Jaguars would not sample the bits correctly. We saw that the bus was taking longer to transition from the dominant to recessive states fast enough when every CAN driver on the bus was driving dominant.
We were able to successfully improve the timing by lowering the resistance of the terminator to 100 ohms. The lower resistance helped pull the CANH and CANL to the recessive state much faster. We also found that minimizing the cable lengths between the Jaguars helped a bit too.
The bottom line:
TI specifies 100 ohm termination resistors and ~20' total of cabling. This differs from the official ISO specification for CAN, but should ensure that teams have the best experience with the Jaguar when using CAN.
-David