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| CANBUS |
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| Sal Ammoniac:
Here's a real-world data point: My weather station mounted on a radio tower sends sensor readings via CAN to my house over 178 feet of terminated CAT-5 twisted pair cable. It runs reliably at 500 kbps, but not at 1 mbps. |
| Siwastaja:
--- Quote from: Sal Ammoniac on October 21, 2020, 08:53:15 pm ---Here's a real-world data point: My weather station mounted on a radio tower sends sensor readings via CAN to my house over 178 feet of terminated CAT-5 twisted pair cable. It runs reliably at 500 kbps, but not at 1 mbps. --- End quote --- Exactly as expected; it's just beyond the recommended 40m, but not so long it would be 100% impossible. It would be interesting to see if you could make it barely work by adjusting the CAN bit timing, especially the sampling point which is typically somewhere around 80% of the bit time but you could make it a bit later, to prove the point re propagation delays. |
| Zero999:
--- Quote from: filssavi on October 20, 2020, 08:01:36 am ---The propagation delay in itself have nothing to do with it unless you plan to somehow run canbus at GHz speed :-DD --- End quote --- Are you confusing logic gate propagation delay, with that of the cable? Theoretically, 1GHz speed CAN would only work over 20cm, so practical purposes would be confined to the same small piece of equipment, perhaps just a single PCB. I doubt you can get line drivers which are fast enough and comply with the standard, but it's possible to make something like CAN, which works at that speed. If you need to transfer data that quickly over a PCB, there are far superior alternatives. |
| amyk:
--- Quote from: David Hess on October 21, 2020, 07:41:54 am ---If the cable is unterminated, then it appears as a capacitive or inductive load depending on frequency. In the common case where the end is open, it appears capacitive at low frequencies with a time constant RC from the series resistance of the driver and total capacitance of the cable which depends on cable length. What is actually happening is the reflections bouncing back and forth charge the cable in steps, with each step lasting for a duration equal to the propagation time through the cable. When the cable is properly terminated, then the reflections are absorbed and this never happens. --- End quote --- Used to advantage in https://en.wikipedia.org/wiki/Reflected-wave_switching |
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