sug­gested bit timing and bus length

Bit­ti­ming as in CiA Draft Stan­dard 102 Ver­si­on 2.0

Bit rate (1) Bus length Nomi­nal Bit time Num­ber of
time quan­ta per bit
Loca­ti­on of sample
point time quanta
Loca­ti­on of sam­ple point
percentage
10 kbit/​s 5000 m (3) 100 µs 16 14 87,5 %
20 kbit/​s 2500 m (3) 50 µs 16 14 87,5 %
50 kbit/​s 1000 m (3) 20 µs 16 14 87,5 %
125 kbit/​s 500 m (2) 8 µs 16 14 87,5 %
250 kbit/​s 250 m (2) 4 µs 16 14 87,5 %
500 kbit/​s 100 m 2 µs 16 14 87,5 %
800 kbit/​s 50 m 1,25 µs 10 8 87,5 %
1 Mbit/​s 25 m 1 µs 8 6 87,5 %

Oscil­la­tor fre­quen­cy : 16 MHz +/-0.1% (1000 ppm)
Sam­pling mode : Sin­gle sam­pling SAM = 0
Syn­chro­ni­za­ti­on mode : Reces­si­ve to domi­nant edges only SYNC = 0
Syn­chro­ni­za­ti­on jump width : 1 * tq SJW = 0
Pha­se Seg­ment 2 : 2 * tq TSEG2 = 1

Note 1 : Roun­ded bus length esti­ma­ti­on (worst case) on basis 5 ns/​m pro­pa­ga­ti­on delay and a total effec­tive ECU-inter­nal (Elec. Con­trol Unit) in-out delay as follows :

  • 1M-800 kbit/​s : 210ns
  • 500 – 250 kbit/​s : 300 ns (inclu­des 2 * 40 ns for optocouplers)
  • 124 kbit/​s : 450 ns (inclu­des 2 * 100 ns for optocouplers)
  • 50 – 10 kbit/​s : 1.5 tq ; Effec­tive delay = delay reces­si­ve to domi­nant plus domi­nant to reces­si­ve divi­ded by two.

Note 2 : For bus length grea­ter than about 200 m the use of opto­coup­lers is recom­men­ded. If opto­coup­lers are pla­ced bet­ween CAN con­trol­ler and trans­cei­ver this affects the maxi­mum bus length depen­ding upon the pro­pa­ga­ti­on delay of the opto­coup­lers i.e. -4m per 10 ns
pro­pa­ga­ti­on delay of employ­ed opto­coup­ler type.

Note 3 : For bus length grea­ter than about 1 km bridge or repea­ter devices may be needed.

FAQ fielbus analysis

In Gene­ral

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