Variable distance b/w CAN nodes on CAN BUS ---- slla279a

Hi,

Will someone like to answer on this thread ?

Regards,

dinesh

Dinesh,

This forum is for Automotive Applications, and this is an Industrial Interface (the SN65HVD255 is not an automotive device) question. I am going to move your post to the correct location. 

But in the mean time, here is my feeback in red:

This application note tells about minimum distance between can node.

•      What does actually this application note says that we canot have variable length cable b/w interconnected can nodes ?

•      The application note is saying that in some severe cases reflections can occur on the CAN bus that can cause communcation errors. This will obviously be system dependent and may or may not be an issue in your system.

•      If variable length cable is there it will cause errors of reflection as shown in figures ?

• Again this depends on how severely the characteristic impedance of the cabling is altered. This will be system dependent.

•      But if physical topology of an plant is as such that we have to install can nodes at varible distance from each other then what to do ?

• When multiple nodes are added close together the capacitance will lower the impedance of the bus and cause reflections. By adding extra bus cabling between these nodes (see difference between Figure 7 and 8 when bus length between each node is changed from 5 inches of cable to 0.5 meters of cable), and therefore adding inductance to the bus, the impedance of the bus will not drop as much. This in turn keeps the characteristic impedance of the bus close to 120Ω and the reflections will be minimized and integrity of the data will be keep. 

For a good approximation, the characteristic transmission line impedance seen into any cut point in an unloaded CAN bus is defined by Z = sqrt(L/C), where L is the inductance per unit length and C is the capacitance per unit length. As capacitance is added to the bus in the form of devices and their interconnection, the bus impedance is lowered to Z'. When the bus impedance is lowered, an impedance mismatch occurs between unloaded and loaded sections of the bus.

• Means if we have cat5 cable then this does not means we have to terminate it 100 ohm resistance & hope that every thing will work corretly. We have to compensate for the loading capacitor to exactly find value of terminating resistor as characteristics Impedance have changed due to loading?

• The CAT5 cable will still have a characteristic impedance, and will therefore need to be terminated to prevent reflections.

Note the change in magnitude when more than one node is sending a dominant bit. The propagation delay of 5 ns per meter for 200 m is 1000 ns, or 1 ms, and is clearly evident in each of the waveforms. 

• what actually happens here ? Signal of Which wave is reflected ---- first node at 200 m or other one ?

• I do not actually know which nodes in the network were sending and where this capture was taken. But, what is happening on a basic level is that multiple nodes are sending the same dominant waveform, but since there are mismatches in the network impedance, reflections are occurring. These reflections cause the waves to meet out of phase which in turn leads to destructive interference between the waveforms. This results in the combined waveform to be reduced in amplitude, below the dominant bit threshold voltage causing a data error. 

Hope this helps,

John

Thanks,John it clarified most of doubts. I will go through ths app note again.

Regards,

Dinesh

Hi,

I have read through specs for SN65HVD255 many times. Actually i just want to confirm some point about SN65HVD255 before finalising it as right sample to use.

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ISO 11898 requires:
Short circuit protection = -3.0V to +32V
Transient protection = -150V to +100V

SN65HVD255 :----
– HBM ESD Protection Exceeds ±12 kV ------- Is this Transient Protection limit ?
– Bus Fault Protection –27V to 40V ------- Is this Short circuit protection limit or DC tolerance limit ?

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As per SLLU158 application note, which contains the circuit for the SN65HVD255 it contains protecting circuit like common-mode chokes & TVS-diodes.
Initially i posted on the canlist forum about protection circuit on can but. I got the following comments.

common-mode chokes :----
Based on my experiences on high-speed networks, using anything else, such as common-mode chokes and so called EMI-capacitors will introduce mor cons than pros.
TVS-diodes :----
>> please keep track on parasitic capacitance - there are significant differences between TVSs of different manufacturers,

 About TVS diode it is ok.  Shall we use common mode chokes in our circuit in this case or not ?

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I read one APPLICATION NOTE from philips AN96116.  What do you suggest about using high speed optocoupler like HCPL-7101 at physical layer ?

Note: If high bit rates shall be used, e.g. 500 kbit/s or above, then high-speed optocouplers
should be considered with a delay of less than 40ns, e.g. HCPL-7101.

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- There are some Fault-tolerant CAN transceiver like MC33388 & TJA1054. Which in case of some can bus fault automatically switch to single wire mode.
But there speed is limited to 125kbps.  As per datasheet SN65HVD255 does not support this. Only support is for CAN BUS SHORT CIRCUIT CURRENT LIMITING.

Does TI have some Fault-tolerant CAN transceiver that support this ?

Regards,

Dinesh