RE: Single ended termination?.

I'll try and reply again but for some reason not all my posts show up.   Th=
ere is lots of information around single terminated transmission lines.  If=
 you remove CAN from searches.  It's simply electrical engineering of trans=
mission lines.

For example.  And maybe this is picking at straws.  The higher end PC board=
 layout software will help you analyze and make sure that when you have bal=
anced transmission lines on the board that the routing is done so that they=
 remain not only balanced but the correct distance apart and above  ground =
plane in order to maintain the impedance of the traces.

So it's critical.

How critical for CAN bus at lower frequencies with the ability to set sampl=
ing at 3x per bit etc. maybe not so much.  But it really gets back to follo=
wing rules because the rules are there due to decades of research on transm=
ission line theory.=20

The paper on distance between nodes and my scope photo bearing that out als=
o shows that there are lots of things at play and they will mostly cause th=
e occasional bump in the error counter.  But one transmission error bumping=
 it by 8 and then 8 good transmissions makes the CAN protocol so robust tha=
t maybe it doesn't matter.

The other thing to keep in mind is that you are in a small cabinet and odds=
 are all the nodes share the same power rail.  That's also not a problem at=
 low frequencies but at higher ones if one node draws enough current to cau=
se voltage shifts on the ground wire you then run into other issues.  Most =
of the far east dongles for example expect common power rail so don't even =
include a ground pin for the CAN bus which IMHO is wrong.    What appears t=
o be low DC resistance may well be high AC impedance at the electrical nois=
e frequencies.

Hope that helps.
John Dammeyer

> -----Original Message-----
> From: [email protected] [mailto:canlist-owner@vector-in=
formatik.com] On Behalf Of Al Thomason
> Sent: February-25-21 8:10 AM
> To: [email protected]; [email protected]
> Subject: RE: [CANLIST] Single ended termination?.
>=20
> All
>=20
> I wish to thank those who replied, and also apologize for me delay in doi=
ng so. CAN knowledge is very limited, all but impossible to
> find another with practical experience.  And to my question, my take away=
 is:  No, there is no definitive study around single ended
> termination of short CAN networks.   We will likely continue our current =
practice of recommending proper dual ended, but also
> recognizing that many will do a single ended install and likely be OK...
>=20
> -al-
>=20
>=20
>=20
> -----Original Message-----
> From: [email protected] [mailto:canlist-owner@vector-in=
formatik.com] On Behalf Of John Dammeyer
> Sent: Wednesday, February 10, 2021 11:33 PM
> To: [email protected]
> Subject: RE: [CANLIST] Single ended termination?.
>=20
> Here's a link to the scope photo.
> http://www.autoartisans.com/CAN/SmplACKDel36m-R-Photo2.jpg
>=20
> Notice the small bump at about 2.1uS.  That's the reflection on a 36m cab=
le at 1Mbps from nodes only 9" apart.  And notice that the
> red trace which is the sum of the differential signal properly removes di=
fferential noise and ringing but the bump gets through.
>=20
> If it's big enough and at the right spot you get the wrong signal.  This =
one appears to be from the stronger ACK accumulated from 50
> nodes.  I've not done further research in this area.
>=20
> John Dammeyer
>=20
>=20
> > -----Original Message-----
> > From: [email protected] [mailto:canlist-owner@vector-=
informatik.com] On Behalf Of John Dammeyer
> > Sent: February-10-21 11:25 PM
> > To: [email protected]
> > Subject: RE: [CANLIST] Single ended termination?.
> >
> > OK,
> > I'll try again with a message I sent on Monday but never showed up.
> > I've removed the photo of CAN reflections as outlined in Steve's paper.
> > I'll post it on a web page.
> > John
> >
> > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> > You know of course the reason you need at least one resistor on the ben=
ch is because the drivers require that return path or they
> just
> > won't work.
> >
> > Now I've run all sorts of bench implementations with only one 120 ohm r=
esistor.  I've also used 300' of the non twisted 4
> conductor
> > solid tan jacketed telephone zip wire for demos but there I did have tw=
o resistors.  So the experience side of me says it will
> probably
> > work.
> >
> > But think about this for a second.  The point of using a transmission l=
ine with 120 ohm impedance and a transmission line
> termination
> > at each end is to absorb reflections and create a bus with square wave =
signals that don't ring and aren't upset up reflections.
> >
> > The actual bit rate you use doesn't matter because the rise time of the=
 signals is still the same regardless of 10kbps or 1Mbps.  The
> > late Steve Corrigan, designer of the TI drivers, wrote an interesting p=
aper on bus spacing along with stub length.  He showed that if
> the
> > nodes are too close together you can get a reflection that superimposes=
 on other data bits.  I had trouble believing that until I took
> a
> > scope photo showing exactly that;  my nodes were too close together.
> >
> > The other problem is your data is somewhat random. You can send a nice =
symmetrical 20kHz square wave through a bus driver
> and
> > look at the signal to see if an open end results in reflections on your=
 bus.  But you aren't sending that sort of data.  It's all over the
> > place with anything from symmetrical to bit stuffing resulting in 5 dom=
inant or 5 recessive broken up by the stuff bit.
> >
> > Again, if there is a reflection that changes the bit level enough to ch=
ange the data value you might not be able to reproduce it but
> > oddly the error counters cycle above 0.  Sometimes once per month to bu=
s recessive or even bus off.  But it's one of those
> problems
> > that a 3 fingered salute on the keyboard fixes.  Or appears to fix.  Or=
 it just goes away.
> >
> > So the short answer is probably yes.  But a human life depended on the =
result would you then follow the rules?
> >
> > I'm rather anal that way so I follow the rules and then I don't have to=
 think about bus issues and focus on how I screwed up my
> > software.
> >
> >
> >
> > From: [email protected] [mailto:canlist-owner@vector-=
informatik.com] On Behalf Of John Dammeyer
> > Sent: February-10-21 11:18 PM
> > To: [email protected]
> > Subject: RE: [CANLIST] Single ended termination?.
> >
> > Test email.
> > John
> >
> >
> > --
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ems to <[email protected]>
>=20
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>=20
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