RS485 – Effect of Splices on 120 Ohm Twisted Pair Cable Impedance


I am currently in the process of designing a wiring loom for a race car control system which must also connect a number of devices via a CAN bus.

This is my first encounter with CAN bus (J1939-15) systems which call for a 120 Ω UTP bus cable terminated by 120 Ω resistors on each end, and my question relates to the correct method for connecting stubs (device cables) to the bus.

Asking around within my industry, the solutions proposed by colleagues are the following:

  1. Strip insulation only and solder stub connections onto the bus without cutting the cable.
  2. Cutting the cable and using Y or T splitter connectors. (Amphenol and Deutsch seem to make J1939 specific Y-splitter connectors. No mention on either product datasheets about impedance matching however.)
  3. Cutting and using 2-1 environmental splices instead of Y-splitter connectors (e.g MIL-S-81824/1 or Raychem MiniSeal splices).

Is there a better or correct way to attach stubs to the main bus?

If not, of the three above options, would the use of the splices linked in the third option adversely affect the network?

Note: In this instance the job specification does not allow me to use solder unless the joint is also encapsulated in a specified potting compound, and I would also prefer avoid the Deutsch Y-splitter connectors due to bulk.

The baud rate is 1 Mbit/s, and bus length is 20 ft (6 m). Stubs in most cases will be less than 1 ft (0.3 m) length.**

Best Answer

"Don't use stubs just loop in and loop out of each module" as Andy Aka said. UTP rigid cable may not be the best solution. Use a shielded twisted pair cable with characteristics impedance 120 ohm. At each device/stub wrap the wires or crimp them, don't solder them. The type of splitters you have mentioned are in my opinion too expensive and they just don't add any benefit to your system, except if you are doing hi-tech military stuff. I have installed many CAN devices in industrial environment, kind of monitoring system. As a base rule, I used a good shielded cable, correct impedance (UNITRONICS BUS CAN) and no stubs. I'd suggest you not to use UTP, as the rigid wire can break when twisting it, also UTP has no shield.


@Conway @Lundin:I agree. I have misunderstood the question about vehicle CAN aka J1939. However, I would like to underline that it would be good if you crimp wires and use some standard vehicle connector where you will join also power wires. As for characteristics impedance it matters the conductor diameter and overall diameter (as well the \$\varepsilon_r\$ of insulating material). I have found some online calculator:

The \$\varepsilon_r\$ of the insulator has a standard value regarding the material. If you need perfect impedance match, you have just to choose a suitable wire and wrap them to make a twisted pair. Alternatively you can also inspect CAN wires from a car at junkyard.


AWG20 = 0.032 in

distance between conductors == overall diameter = 0.085 in

dielectric constant PVC = 3

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