RE: switchable or separate termination in can network

My personal preference would be to hunt down the designer of the nodes with
soft-switchable terminators for a very stern talking-to on why that is never
a clever idea. (unless we are talking about FT-CAN, where switchable
termination is part of the deal). Hard-switchable terminators
(jumpers/cables) should be measurable with a powered-down network, so that
should not matter.

In practice, I don't see an easy way to detect soft-switchable termination
unless you have the proper equipment (which ranges from not-so-cheap to
eye-wateringly-expensive) and skills. In the real world, telematics
installers get a multimeter and a flowchart, and maybe a way to determine if
the CAN network is working (diagnostics output of the node they are
installing). So your best bet would be: follow the "regular" directions, see
if the CAN network functions afterwards and if not: invert the termination
setting of the device you installed to see if that fixes things. If that
doesn't work, bring in the fancy equipment and engineers, start
unplugging/unpowering nodes from the network, and spend the best part of the
day trying to figure out what is happening.

cheers,
Bram
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf
Of Hadarits Kft.
Sent: woensdag 24 juni 2015 21:58
To: [email protected]
Subject: [CANLIST] switchable or separate termination in can network

Hi All,

we  install fleet managenet devices in vehicles and sometimes connect its to
the CAN system too. We need to keep the termination value on 120ohm. The
device's installation manual offer to measure (with multimeter) the
resistance between the CANL and CANH wires (without powering the CAN
network; in general switch off the ignition). Depends on the measured value
we have to swith on or off the device's termination resistor.
My question: if the CAN network consists one or two node with switchable
termination resistor at the network's end, will the measurement (without
powering) shows the real network termination resistance? In case it won't
shows the real value, is there any easy way to determine it?  (or I have to
use scope to  evaluete the rise and drop edges?)

Thanks in advance: Tamas Hadarits
www.tachografszerviz.hu





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