--- In 6..., "Darren Moore" wrote:
> The data will pass without the ground, this is not
the
> full CAN spec, which includes power down and other control
> modes used with the lines not in the normal common mode
> operation. So it depends on what you want from the data
> link as to if it will do all you need and at what data
> rates.
>
> The full CAN spec from Bosch has all this information in
> it.
>
> Collision detection is done by two nodes pulling the bus
> active at the same time, who ever releases the bus first
> loses and tries again later, there is no data loss for
> the other talker.
> All the drivers on the bus are OR'd
> together as the drivers are open collector outputs.
Yes but there is a twist, the CANH and CANL are driven in opposite directions,
i.e. one is driven from a source the other from a sink.
The result is that they are driven together in opposite directions to develop
the differential voltage aross the terminating resistor.
Somewhat different from the standard differential pair where the lines are
driven symmetrically.
Cheers,
Theo
why CAN bus has two lines (CAN_L and CAN_H)
Started by ●May 16, 2007
Reply by ●May 31, 20072007-05-31
Reply by ●May 31, 20072007-05-31
--- In 6..., "theobee00" wrote:
> Sorry for that, I did not want to imply that you are a hacker, I
> know better.
>
> It was just a small dig at you when you failed to explain why the
> particular chip you use could work without ground when you did your
> test.
Hey Theo, thanks. I was hoping you'd respond about the hacker thing
;-) However I see nothing left out in my explanations, except that I
don't use the same terminology you do. So it is you who failed to
understand my explanation. It's quite intriquing though, isn't it, how
connecting one or two random wires between two otherwise isolated
circuits makes a new common ground for the new circuit including them
both, isn't it?
> As I said at the time it either was possible because the bus was
> driving protective devices or it relied on active circuits to
> achieve this, I observed at the time there was an amazingly small
> difference between the "soft" grounds.
>
> You now forced me to delve into the specific device you used which I
> tried to avoid since I prefer spending my time arguing theory
>
> It turns out that the specific Philips device you use? has a
> reference generator build in, it says it sets the differential
> outputs to half rail voltage, I.e 2.5 Volts, since the bus drivers
> are 180 degrees out of phase, the average is always 2.5 Volts, the
> receivers in turn have a resistor to the ref Voltage, I quote;
I did catch your previous comments about protective or active circuits
and was trying to get back to that because of course that is what I
was trying to say that it is... which means of course that OP may not
need to send the common Gnd with it. In fact, I think it was a
Freescale SR where they told me that I did not need the Gnd
connections on the CAN cable. I avoided saying that because I don't
want to look it up to verify it, but I think it was (I have terrible
memory except for certain technical details that I think important).
So anyway, victory! My whole plot was to force you to look at the
specific hardware and not just talk general theory :)
I don't know if you noticed, but I believe the original question was
from a less experienced (probably newbie) asking about the Gnd pins on
their CAN header on either a Freescale evaluation board, or similar
clone implementation (Dragon12, Techarts, etc). I believe the poster
did not specify the 82C250 because of the assumption that it was the
only way to implement a CAN bus. Also note that it is not only
Phillips. I heard that their proprietary rights to this nifty CAN
driver have expired.
> Sorry for that, I did not want to imply that you are a hacker, I
> know better.
>
> It was just a small dig at you when you failed to explain why the
> particular chip you use could work without ground when you did your
> test.
Hey Theo, thanks. I was hoping you'd respond about the hacker thing
;-) However I see nothing left out in my explanations, except that I
don't use the same terminology you do. So it is you who failed to
understand my explanation. It's quite intriquing though, isn't it, how
connecting one or two random wires between two otherwise isolated
circuits makes a new common ground for the new circuit including them
both, isn't it?
> As I said at the time it either was possible because the bus was
> driving protective devices or it relied on active circuits to
> achieve this, I observed at the time there was an amazingly small
> difference between the "soft" grounds.
>
> You now forced me to delve into the specific device you used which I
> tried to avoid since I prefer spending my time arguing theory
>
> It turns out that the specific Philips device you use? has a
> reference generator build in, it says it sets the differential
> outputs to half rail voltage, I.e 2.5 Volts, since the bus drivers
> are 180 degrees out of phase, the average is always 2.5 Volts, the
> receivers in turn have a resistor to the ref Voltage, I quote;
I did catch your previous comments about protective or active circuits
and was trying to get back to that because of course that is what I
was trying to say that it is... which means of course that OP may not
need to send the common Gnd with it. In fact, I think it was a
Freescale SR where they told me that I did not need the Gnd
connections on the CAN cable. I avoided saying that because I don't
want to look it up to verify it, but I think it was (I have terrible
memory except for certain technical details that I think important).
So anyway, victory! My whole plot was to force you to look at the
specific hardware and not just talk general theory :)
I don't know if you noticed, but I believe the original question was
from a less experienced (probably newbie) asking about the Gnd pins on
their CAN header on either a Freescale evaluation board, or similar
clone implementation (Dragon12, Techarts, etc). I believe the poster
did not specify the 82C250 because of the assumption that it was the
only way to implement a CAN bus. Also note that it is not only
Phillips. I heard that their proprietary rights to this nifty CAN
driver have expired.
Reply by ●June 1, 20072007-06-01
--- In 6..., "Jefferson Smith" wrote:
>
> --- In 6..., "theobee00" wrote:
> > Sorry for that, I did not want to imply that you are a hacker, I
> > know better.
> >
> > It was just a small dig at you when you failed to explain why the
> > particular chip you use could work without ground when you did your
> > test.
>
> Hey Theo, thanks. I was hoping you'd respond about the hacker thing
> ;-) However I see nothing left out in my explanations, except that I
> don't use the same terminology you do. So it is you who failed to
> understand my explanation.
That would be bloody right, sme again.
> It's quite intriquing though, isn't it, how
> connecting one or two random wires between two otherwise isolated
> circuits makes a new common ground for the new circuit including them
> both, isn't it?
Old principle, same drawback and problems as before, high impedance ground, lots of common mode noise, I must admit this Philips chip allows for some plenty of headroom.
> > As I said at the time it either was possible because the bus was
> > driving protective devices or it relied on active circuits to
> > achieve this, I observed at the time there was an amazingly small
> > difference between the "soft" grounds.
> >
> > You now forced me to delve into the specific device you used which I
> > tried to avoid since I prefer spending my time arguing theory
> >
> > It turns out that the specific Philips device you use? has a
> > reference generator build in, it says it sets the differential
> > outputs to half rail voltage, I.e 2.5 Volts, since the bus drivers
> > are 180 degrees out of phase, the average is always 2.5 Volts, the
> > receivers in turn have a resistor to the ref Voltage, I quote;
>
> I did catch your previous comments about protective or active circuits
> and was trying to get back to that because of course that is what I
> was trying to say that it is... which means of course that OP may not
> need to send the common Gnd with it. In fact, I think it was a
> Freescale SR where they told me that I did not need the Gnd
> connections on the CAN cable. I avoided saying that because I don't
> want to look it up to verify it, but I think it was (I have terrible
> memory except for certain technical details that I think important).
>
> So anyway, victory! My whole plot was to force you to look at the
> specific hardware and not just talk general theory :)
Caught again, got me consulting for free again:-)
Do take into consideration that it is designed for cars where the whole body acts as a grounded plane with very little DC ofsets, the problems you get would be mainly RF related.
> I don't know if you noticed, but I believe the original question was
> from a less experienced (probably newbie) asking about the Gnd pins on
> their CAN header on either a Freescale evaluation board, or similar
> clone implementation (Dragon12, Techarts, etc). I believe the poster
> did not specify the 82C250 because of the assumption that it was the
> only way to implement a CAN bus. Also note that it is not only
> Phillips. I heard that their proprietary rights to this nifty CAN
> driver have expired.
Can't see how they can have serious rights to a line driver, these have been around since I was a kid, around the same era as Methusalem.
Now back to that soup recipy...
Cheers,
Theo
>
> --- In 6..., "theobee00" wrote:
> > Sorry for that, I did not want to imply that you are a hacker, I
> > know better.
> >
> > It was just a small dig at you when you failed to explain why the
> > particular chip you use could work without ground when you did your
> > test.
>
> Hey Theo, thanks. I was hoping you'd respond about the hacker thing
> ;-) However I see nothing left out in my explanations, except that I
> don't use the same terminology you do. So it is you who failed to
> understand my explanation.
That would be bloody right, sme again.
> It's quite intriquing though, isn't it, how
> connecting one or two random wires between two otherwise isolated
> circuits makes a new common ground for the new circuit including them
> both, isn't it?
Old principle, same drawback and problems as before, high impedance ground, lots of common mode noise, I must admit this Philips chip allows for some plenty of headroom.
> > As I said at the time it either was possible because the bus was
> > driving protective devices or it relied on active circuits to
> > achieve this, I observed at the time there was an amazingly small
> > difference between the "soft" grounds.
> >
> > You now forced me to delve into the specific device you used which I
> > tried to avoid since I prefer spending my time arguing theory
> >
> > It turns out that the specific Philips device you use? has a
> > reference generator build in, it says it sets the differential
> > outputs to half rail voltage, I.e 2.5 Volts, since the bus drivers
> > are 180 degrees out of phase, the average is always 2.5 Volts, the
> > receivers in turn have a resistor to the ref Voltage, I quote;
>
> I did catch your previous comments about protective or active circuits
> and was trying to get back to that because of course that is what I
> was trying to say that it is... which means of course that OP may not
> need to send the common Gnd with it. In fact, I think it was a
> Freescale SR where they told me that I did not need the Gnd
> connections on the CAN cable. I avoided saying that because I don't
> want to look it up to verify it, but I think it was (I have terrible
> memory except for certain technical details that I think important).
>
> So anyway, victory! My whole plot was to force you to look at the
> specific hardware and not just talk general theory :)
Caught again, got me consulting for free again:-)
Do take into consideration that it is designed for cars where the whole body acts as a grounded plane with very little DC ofsets, the problems you get would be mainly RF related.
> I don't know if you noticed, but I believe the original question was
> from a less experienced (probably newbie) asking about the Gnd pins on
> their CAN header on either a Freescale evaluation board, or similar
> clone implementation (Dragon12, Techarts, etc). I believe the poster
> did not specify the 82C250 because of the assumption that it was the
> only way to implement a CAN bus. Also note that it is not only
> Phillips. I heard that their proprietary rights to this nifty CAN
> driver have expired.
Can't see how they can have serious rights to a line driver, these have been around since I was a kid, around the same era as Methusalem.
Now back to that soup recipy...
Cheers,
Theo
