Re: can network establishing -- Can bit timing

Hello Eric,

Propagation delay consists of:
- Propagation delays of transceiver: TX-to-CAN _and_ CAN-to-RX
- Propagation delay of logic, if additional logic exist between controller and transceiver
- Propagation delays of isolation, if such is used
- Signal propagation delay required for signal to go from end to end of the network _and_
  back. Propagation delay needs to be taken as twice, because during arbitration and ACK,
  network state may be driven by multiple devices and there shall be enough time to enable
  network state to stabilize before taking a sample.

Some example values:
- TJA1050: TX-to-CAN worst case 110ns, CAN-to-RX worst case 155ns
- Logic depends on the implementation, but fast CPLDs such as Lattice ispM4A provide <10ns
- Best option is to use digital isolators, e.g. for ADuM1100 worst-case 18ns
- Signal propagation delay is 5ns/m in standard 120ohm CAN-cable

=> Basic implementation without isolation and logic, we get:
   1: At 1Mbps CANopen we have 4TQ=500ns reserved for propagation delay in a worst case
   2: 110ns + 155ns = 255ns for transceiver -- 195ns is left for signal propagation
   3: We can achieve maximum cable length of 195ns/(2*5ns/m)=19.5m

   4: If we have the best possible 5TQ=625ns propagation segment
   5: Maximum cable length of 320ns/(2*5ns/m)=32m may be achieved

=> The basic timing is only one part of the truth. Additional constraints are required for
   the topology to keep the transmission line solid:
   - Few group of devices far away from each other should be avoided, because the groups
     modify the line-impedance in the area of the group => impedance junctions introduce
     reflections
   - In addition to previous, residual error probability decreases significantly when the
     number of devices are increased -- use numerous devices, but try to install them evenly
     along the network.
   - Too long drop-lines are unterminated ends, causing reflections => the longer drop-line,
     the closer to sample point the reflection can appear. The more there are long drop-lines,
     the higher power reflections appear

That was timing in brief. I hope you got some help.

Best regards,

-H


TK Engineering Oy
Heikki Saha, CTO
  M.Sc. Automation
  Dr.Tech. Electronics
Mail address:  P.O. box 810, FIN-65101 VAASA
Visit address: Yrittäjänkatu 15, FIN-65380 VAASA
+358 (0)50 588 6894
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http://www.canopen.fi/

----- Original Message -----
From: "Eric clawson" <[email protected]>
To: [email protected]
Sent: Friday, October 18, 2013 1:21:57 PM
Subject: Re: [CANLIST] can network establishing -- Can bit timing


Thanks Bertil, 


For this nice calculator. But i need to estimate exact propagation segment delay. 


1> But my question is related to estimating the length of the cable to find exact propagation delay . Theoretically length of the cable will be : drop down length(combined for all nodes) + actual length 


2> Mostly delay will also because of drop down connectors & also trans-receivers . 


3> So when estimating total length of network then it will be : 
drop down length(combined for all nodes) + actual length ? 



4> Also how much maximum can be drop down length(combined for all nodes) ? 


Any suggestion..!! 





On Fri, Oct 18, 2013 at 11:32 AM, bertil bäck < [email protected] > wrote: 


Hi, 
have a look here: 
http://www.bittiming.can-wiki.info/ 

For Freescale msCAN (HCS12) use SJA1000 

Br, 
Bertil Bäck 
R&D Manager, Hardware 
TK Engineering 



----- Original Message ----- 
From: "Eric clawson" < [email protected] > 
To: [email protected] 
Sent: Thursday, October 17, 2013 8:18:40 PM 
Subject: [CANLIST] can network establishing -- Can bit timing 



Hi, 

I am working on a CAN network where i have 20 nodes attached to my network. 

Speed = 125 Kbps 
Clock = 8 MHZ (I can increase it further if required) 
Nodes = 20 
Cable = 120 ohm characteristics impedance. 
Drop Length per node = 2 meter 
Total length of cable network = 80 Meter 

1> Now how can i set the Bit timing for my CAN network so that with this topology it works at 87.5% sample point ? 
One experience person told me at 125 KBPS we can cover :-- 

* 500m length total. 
* Double the length of drop down cable. 
* Maximum length of drop should be 120M 
* Also for every connector consider 6 meter loss in length. 



2> Please suggest is above calculation right? 
3> If not then what eactly should i keep the value of propagaton segment of my network? 
4> Also what value to be kept for phase segment 1 & 2 ? 

I am also refering an1798.pdf for it. But i need some to share his experience in setting CAN network . 

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