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Unread 12-05-2010, 11:38
Unsung FIRST Hero
Al Skierkiewicz Al Skierkiewicz is offline
Broadcast Eng/Chief Robot Inspector
AKA: Big Al WFFA 2005
FRC #0111 (WildStang)
Team Role: Engineer
 
Join Date: Jun 2001
Rookie Year: 1996
Location: Wheeling, IL
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Re: Using a star configuation for Jaguars on CAN

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.
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Good Luck All. Learn something new, everyday!
Al
WB9UVJ
www.wildstang.org
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Storming the Tower since 1996.

Last edited by Al Skierkiewicz : 12-05-2010 at 11:43.