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can_cabling [2013/03/26 15:09] – added cable length discussion heinz | can_cabling [Unknown date] (current) – removed - external edit (Unknown date) 127.0.0.1 | ||
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- | ==== CAN cabling ==== | ||
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- | Some rules on how to design a CAN network. Consider reading [[: | ||
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- | The rules are general and valid for an CAN network including [[: | ||
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- | * Always use twisted pairs for CAN_H and CAN_L. | ||
- | * Several manufacturers offer also four twisted cables for CAN when using 4-wire construction. This makes cable thinner and enables moulding of 2x2x0.5mm2 into e.g. M12-connectors | ||
- | * Maximum cable length is between the two farthermost nodes on the CAN bus line. Remember, in the worst case a signal has to travel from the node at one end to the node on the other and of the line. It is a function of the bit rate as follows: | ||
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- | ^bit rate ^max cable length (in m) ^ | ||
- | | 1Mbit/s | 25 | | ||
- | | 800Kbit/s | 50 | | ||
- | | 500Kbit/s | 100 | | ||
- | | 250Kbit/s | 250 | | ||
- | | 125Kbit/s | 500 | | ||
- | | 50Kbit/s | 1000 | | ||
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- | (Table from CiA DS 301 4.02 page 20) \\ CiA 303 Part 1: Cabling and connector pin assignment has recommendations for connectors and cables used in CANopen systems. | ||
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- | If you ask for a formula? There is one rule of thumb: | ||
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- | [[http:// | ||
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- | ==== CAN cable types ==== | ||
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- | All big vendors have specialized CAN cables, but they are not necessary called **CAN** | ||
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- | * [[http:// | ||
- | * [[http:// | ||
- | * [[http:// | ||
- | * [[http:// | ||
- | * [[http:// | ||
- | * [[http:// | ||
- | * [[http:// | ||
- | * [[http:// | ||
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- | ==== The maximum | ||
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- | An interesting discussion from the CANLIST: | ||
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- | ''' | ||
- | According to the DS303-1 and the Bosch protocol and ISO 11898, the 1Mbit should be supported for bus length of 30meters. Practically we could reach not more than 10meters. This also stands with some notes that I could locate on the web. | ||
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- | Does anyone have any experience to share in such architecture? | ||
- | Any suggestion in how can a bus be lengthen to 30meters? Any idea what were the different standrad calculations when allowing 30-40 meters? | ||
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- | ''' | ||
- | Did you have done any calculations of your overall signal propagation, | ||
- | transceiver - opto coupler - cable - opto coupler - transceiver | ||
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- | As you already said, having all bad components, you will not reach 30m, in the worst case your calculation will end up with a length of **minus** some meters. | ||
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- | Repeaters will not extend cable length when you stay with 1Mbit/s. They will add additional delay. | ||
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- | ''' | ||
- | A 30 meter bus should not be a problem with a 1 Mbps signaling rate as long as the cable is at the very least 120ohm CAT 5 twisted-pair cable. | ||
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- | Problems you may be having | ||
- | * correct termination? | ||
- | * too many stubs? | ||
- | * stub length too long? | ||
- | * stubs too close together? | ||
- | * trying to use galvanic isolation? | ||
- | * trying to use a non-standard topology like a star? | ||
- | 40m at 1Mbps on a standard bus is no trick at all, so something is not electrically correct with your bus arrangement. This is how your bus should be set up for high-speed operation. | ||
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- | ''' | ||
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- | ''' | ||
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- | ''' | ||
- | taking the worst case scenario according to the manufacturer you might end up with cable with minus length. The problem is that you cannot get the typical values of the delay and not even some statistics of "how many components might get to the max delay out of 1000" | ||
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- | ''' | ||
- | with fast couplers and transceivers I see no problem to make a 100% design. ADuM1100 has typ. 10.5 and max. 18ns prop. delay. HCPLx710 typ. 20ns, max. 40ns. Two times transceiver delay (each 120ns..175ns). Still enough room for several meters of cable. | ||
- | Regarding the probability: | ||
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- | AFAIK galvanic isolation suppresses only low frequency noise, RF can be filtered better and cheaper. \\ Oliver | ||