Re: CAN bit timing, Seg2 =1Tq

Thanks for the detailed explanations.
I am not sure but my understanding is that you are concerned mostly from
the signal distortion due to the length of the line, distortion that may
cause more delay in the sampling time.
Does 20 m line would be acceptable for the above mentioned system?
I found that 100 OHM cable save about 20% in delay time. The Dominant level
of CAN drivers driving  50 OHM load instead of 60 OHM almost the same. What
is your opinion of using 100 OHM line?

Regards,
ab

‫בתאריך יום ו׳, 15 ביולי 2016 ב-19:07 מאת ‪John Dammeyer‬‏ <‪
[email protected]‬‏>:‬

> It's been many years since I've worked on the edge like that.  I found
> that with 35m Beldon Thick DeviceNet and 1MBps that I had to use at least
> 16 Tq per bit to sample at the 87.5% point.
>
> As I recall you need to take into account that a node at the far end may
> not detect the edge of the start bit until the end of its 125n/S clock
> window where the transmitter sends the dominant on the edge of its 8Mhz
> clock.  So you have a potential skew of almost 125nS.  That can put the
> point where the node tests as far out as 124nS which is too late.
>
> Some CAN devices have other kinds of Start Bit Edge Detection logic that
> my synchronize differently but if you think of a simple state machine that
> every 125nS looks at the signal, it may not detect the edge until 124nS
> after it was sent.  So you have almost a 1 bit skew on the signal not
> including the propagation time.
>
> Having said that you can try it.  We found for the 35m segments we had to
> back down our network to 500kbps.  Although it required reprogramming and
> reinstalling 50 nodes, it was easy to do because the TKE Bridge we used
> could have each 50 node segment running at separate bit rates.  We fed it
> with 500kbps, it distributed the messages out the other three ports at
> 500kbps or 1Mbps so we never overran the bridge.
>
> John Dammeyer
>
>
> From: [email protected] [mailto:[email protected]] On
> Behalf Of Amitai Broyde
> Sent: July-15-16 3:45 AM
> To: [email protected]; [email protected]
> Subject: Re: [CANLIST] CAN bit timing, Seg2 =1Tq
>
> Hi,
> The cable is a controlled 120 OHM cable (5nS/m) by Axon Cables . Don't
> recall the exact p/n. I would like to extend the bus to about 30m.
> The dominant to recessive loop worst case delay is 400nS, there are 16
> nodes nodes. All nodes clocks' use XOs - several ppm accuracy.
> What the risk of edgy sampling? it is still 125nS in.
>
> Regards,
> ab
>
>
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