RE: CAN bit timing, Seg2 =1Tq

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 


--
Archives and useful links: http://groups.yahoo.com/group/CANbus
Subscribe and unsubscribe at www.vector.com/canlist/
Report any problems to <[email protected]>