RE: Single ended termination?.

Here's a link to the scope photo.
http://www.autoartisans.com/CAN/SmplACKDel36m-R-Photo2.jpg

Notice the small bump at about 2.1uS.  That's the reflection on a 36m cable=
 at 1Mbps from nodes only 9" apart.  And notice that the red trace which is=
 the sum of the differential signal properly removes differential 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 on=
e appears to be from the stronger ACK accumulated from 50 nodes.  I've not =
done further research in this area.

John Dammeyer


> -----Original Message-----
> From: [email protected] [mailto:canlist-owner@vector-in=
formatik.com] On Behalf Of John Dammeyer
> Sent: February-10-21 11:25 PM
> To: [email protected]
> Subject: RE: [CANLIST] Single ended termination?.
>=20
> 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
>=20
> =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 bench=
 is because the drivers require that return path or they just
> won't work.
>=20
> Now I've run all sorts of bench implementations with only one 120 ohm res=
istor.  I've also used 300' of the non twisted 4 conductor
> solid tan jacketed telephone zip wire for demos but there I did have two =
resistors.  So the experience side of me says it will probably
> work.
>=20
> But think about this for a second.  The point of using a transmission lin=
e with 120 ohm impedance and a transmission line termination
> at each end is to absorb reflections and create a bus with square wave si=
gnals that don't ring and aren't upset up reflections.
>=20
> The actual bit rate you use doesn't matter because the rise time of the s=
ignals is still the same regardless of 10kbps or 1Mbps.  The
> late Steve Corrigan, designer of the TI drivers, wrote an interesting pap=
er on bus spacing along with stub length.  He showed that if the
> nodes are too close together you can get a reflection that superimposes o=
n other data bits.  I had trouble believing that until I took a
> scope photo showing exactly that;  my nodes were too close together.
>=20
> The other problem is your data is somewhat random. You can send a nice sy=
mmetrical 20kHz square wave through a bus driver and
> look at the signal to see if an open end results in reflections on your b=
us.  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 domin=
ant or 5 recessive broken up by the stuff bit.
>=20
> Again, if there is a reflection that changes the bit level enough to chan=
ge the data value you might not be able to reproduce it but
> oddly the error counters cycle above 0.  Sometimes once per month to bus =
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 i=
t just goes away.
>=20
> So the short answer is probably yes.  But a human life depended on the re=
sult would you then follow the rules?
>=20
> I'm rather anal that way so I follow the rules and then I don't have to t=
hink about bus issues and focus on how I screwed up my
> software.
>=20
>=20
>=20
> From: [email protected] [mailto:canlist-owner@vector-in=
formatik.com] On Behalf Of John Dammeyer
> Sent: February-10-21 11:18 PM
> To: [email protected]
> Subject: RE: [CANLIST] Single ended termination?.
>=20
> Test email.
> John
>=20
>=20
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