1
\$\begingroup\$

Normally, I'd advocate use of shielded twisted pair for CANBUS networks but there have been some questions recently about using unshielded cabling to achieve the same aim.

From what I can see there are some options available on how to "construct" unshielded twisted cabling and these are:

a. Three conductors. CANH, CANL and "drain", which are all twisted together, with 30 twists per metre. e.g. Get three wires in a hand drill, attach the other ends in a vice and twist them so that they interleave.

b. Three conductors. CANH, CANL and a "drain"; CANH and CANL are twisted together as for a normal twisted pair but the drain wire runs down the centreline of the twisted pair.

c. Three conductors. CANH, CANL and a "drain"; CANH and CANL are twisted together and the drain runs external to the twisted pair.

Which is the best option of these 3 in terms of being closest to match the shielded twisted pair in performance?

Are there any pros and cons of these 3 approaches?

Is termination of this CANBUS type via the normal 120R?

For the drain connection, should this be connected to a single star point only or is it OK to connect it to one or more ECUs if they have a capacitively coupled path (1R + 68uF) to ground?

I imagined that option b might be the best approach to allow for common mode noise in the twisted pair that would be subtracted at the CAN transceiver but I'm open to being corrected.

Is there anything to particularly look out for when using unshielded cabling, such as limiting the number of nodes, nodes spacing, stub lengths, etc.? I believe that there's a notional limit of 10 nodes where shielded twisted pair is not used but I'm not sure what kind of validation is required to be performed if more than 10 nodes is required.

\$\endgroup\$
7
  • \$\begingroup\$ I guess it depends on in which environment you want the CAN bus to work in. On a few meter bus on an office table, or longer bus in an industrial setting, or near arc furnaces. Also simply twisting the cable means you simply get a twisted pair which means it cancels out external magnetic fields. External electric fields still affect both wires which may be OK in a differential bus but it would be stopped by the shield. Twisting random wires together does not give you a controlled impedance of 120 ohms, but something else or random based on wire geometry. Can you even use Ethernet/CAT5 cable? \$\endgroup\$
    – Justme
    Commented Jan 11 at 20:30
  • \$\begingroup\$ @Justme Thanks for the response. The environment is a vehicle so it is subject to EMI. The wires are standard PVC, 0.5mm2, which I think has a dielectric constant between 4 and 6; I think that the characteristic impedance actually comes out at around 68R, for a 1.6mm separation, which isn't good. \$\endgroup\$
    – komandirskie
    Commented Jan 12 at 8:18
  • 1
    \$\begingroup\$ It is more important to have control over what you are transmitting and take other measures against EMI such as filters, slope control and TVS. Picking a shielded cable should be one of the last resorts, not the first thing you consider. I rarely ever use shield or twisted pair, working almost exclusively with industrial, automotive or military environments. \$\endgroup\$
    – Lundin
    Commented Jan 12 at 15:15
  • 1
    \$\begingroup\$ Also wth is a "drain"? Do you mean signal ground? \$\endgroup\$
    – Lundin
    Commented Jan 12 at 15:16
  • \$\begingroup\$ @Lundin. You're using CAN in those applications and not always using twisted pair? In the case of the filters, slope control and TVS, I presume that these are usually situated within a CAN node itself (ECU) but what would be your normal approach for nodes not containing such protection. Yes, by "drain" I mean a conductor normally connected to a single star point but which, if the ECU supports the correct arrangement, the ECU CAN "shield" connection. \$\endgroup\$
    – komandirskie
    Commented Jan 15 at 7:52

0

Reset to default

Browse other questions tagged or ask your own question.