I connected three devices in parallel on a CAN bus system. Every device has a \$120\,\mathrm{\Omega}\$ resistor built into the circuit. I am using two pairs of twisted cables of \$1.2\,\mathrm{m}\$ each for the communication. Will it have any effect on the communication?
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Data rate? Disposition of the three units? – Andy aka Oct 19 '15 at 07:40
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Data rate is 19600 bps. The units are connected in parallel to each other with with a twisted pair cable of 1.2mtrs between them. – Venigalla Praveen Oct 19 '15 at 07:53
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Assuming that the data receivers are capable of working with the smaller signal (which I'm sure they are), the only aspect that might cause problems are signal reflections.
Given that data rate is 19,600 bps, if you said this was a square wave of frequency 10 kHz, I'd be considering the wavelength of the 7th harmonic in order to justify that the cable length was OK.
7th harmonic is 70 kHz and this has a wavelength of 4.3 km. General rule of thumb is that a badly terminated cable will be OK if the length of the cable is below one-tenth of the highest wavelength and clearly it is.
Further reading: Why does the CAN bus use a 120 ohm resistor as the terminating resistor and not any other value?
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You need to have a 120Ω resistor at the beginning of the chain and at the end of the chain. If one is missing, it won't work (trust me!). If you have too many, it might (MIGHT) work, but you're asking for trouble. CAN is intended to be a linear chain with termination resistors at each end.
Best to figure out a way to get rid of the middle termination resistor. Sometimes this is just a jumper, but you might want to desolder or clip it if that is necessary.
Daniel
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Hi Daniel, Thanks for the reply. But When i communicate with three devices in parallel there is no issue but sometimes i see the communication is broken and again after some time its restoring itself do you think its because of the extra resistance. – Venigalla Praveen Oct 21 '15 at 07:40
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You should use an oscilloscope and try to divine if the signal looks like it is loaded down too much. This might look like the on-states not reaching their full voltage. Honestly I've not tried to troubleshoot this scenario, so I'm not sure how it would show up. Try scoping without the middle device and see if there is a noticeable difference in the waveform such as rise time or peak-to-peak voltage. – Daniel Oct 21 '15 at 22:19
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The more resistors you have, the bigger a load there is on the transceiver trying to drive the bus. If, for example, you had ten 120-ohm resistors, each transceiver will have to deliver 5x the normal current. This will make them heat up, make them unable to complete proper transitions, or both.
Three resistors instead of two may not be the end of the world. But it's still stressing the transceivers unnecessarily.
Stephen Collings
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Hi Stephen, thanks for the reply. Does this extra resistance will create a make break kind of situation – Venigalla Praveen Oct 21 '15 at 07:43
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That's going to depend entirely on the transceivers involved. – Stephen Collings Oct 21 '15 at 12:16