tapping wires directly

[Paul Stafford]

Hi all

Just collecting all the items needed to start sniffing my F650GS twin CANBus

I just bought a CSS electronics CL1000 sniffer, and will use a laptop running wireshark

I was hoping to use what I thought was an unused alarm connector, but as I also have TPS, the connector is in use.

Is there an impedance./reflection or other problem tapping the wires directly behind the connector? Any tap wire length limitations?

I'd expect not, as the CSS sniffer obviously has a 120 ohm termination, and with a 500k bus speed, I would not expect impedance mismatch to be a big issue - but I could be wrong!

Thanks very much

Paul Stafford

Missoula, MT

[Jake Aalfs]

Hi Paul,

Congrats on getting the equipment you need. The CAN bus needs to be terminated at each end with a 120 ohm resistor. If you are tapping into a bus that already has terminating resistors at each end, then you do NOT need a terminating resistor at your attached node.

Jake

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[Paul Stafford]

Hi Jake

thanks for the reply. Are there any limits on stub length?

my stub would terminated with the CL 1000

[Jacob Aalfs]

Paul,

I'm sure there is a limit on stub length. I don't know the limit offhand. It's probably covered in the CAN bus specifications. Here's a link to more information:

https://en.wikipedia.org/wiki/CAN_bus

Jake

[Paul Stafford]

Hi Jake,

Thanks for that- the Wiki didnt have the info, but this doc  from Texas Instruments did.

0.3 m max unterminated stub length

The same doc also was helpful in specing the wire to be used - shielded twisted pair.