Connecting CANOpen Equipment Using RJ45 Connectors and Cabling

Why Use RJ45?

For normal environments, I like using RJ45 cables to connect my CANOpen equipment. Why?

  • Quality cables are readily available - just use Ethernet cables.
  • The connector's latch keeps the cables connected, but is a lot quicker to use than screwing/unscrewing DB9 thumbscrews.
  • Many CANOpen drives come with dual RJ45 jacks, which makes daisy chaining equipment easy (much much easier using single DB9M connectors or doing custom cables with poke & crimp connectors).

Standard Pin Outs

The CAN in Automation document DS303 Part 1 specifies the pin outs for RJ45 connects.

Making the Connection

I like to daisy chain - CANOpen does best with a single main trunk line and very short stub lines (at 1M bits/sec, the trunk cables have to be very short). According to DS303 Part 1 all doubled connections such as Tees or duplex connectors should connect all lines, so a CANOpen daisy chain will work fine even if some of the equipment is turned off.

If all the equipment has dual RJ45 jacks, life is easy. Just start with a RJ45 terminator plug, daisy chain chain the equipment together with Ethernet cables, and then finish with another RJ45 terminator plug.

With mixed connectors, you can convert everything to RJ45 (my approach) or convert the RJ45 to another connector.

  • If a CANOpen device only has one RJ45 jack (such as the Ixxat USB-to-CAN compact with RJ45 jack), you can use a RJ45 duplex adapter (Tee) to convert it to two jacks.
  • If a CANOpen device only has a DB9M connector it is easy to convert to a RJ45 jack.

RJ45 Terminators

I haven't found a good source of pre-made RJ45 terminators. But they're easy to make:

  1. Take a TIA T-568B RJ45 patch cable and cut it close to the connector (if you don't use a T-568B cable, you'll have to check the color scheme).
  2. Strip off the outer insulation.
  3. Cut off all the wires except 1 (orange/white) and 2 (orange).
  4. Put some heat shrink tubing onto wires 1 and 2.
  5. Crimp a 120 Ohm resistor (I used a 1/4 watt) to wires 1 (orange/white) and 2 (orange)
  6. Move the heat shrink tubing so it covers up any bare wires, and use a heat gun to shrink it.
  7. Put some larger heat shrink tubing over the wires and resistor, and use a heat gun to shrink it.
  8. As always, test the result.
RJ45 Terminator Plug

RJ45 Troubleshooting

It's simple to make a RJ45 breakout plug so you can easily use an oscilloscope to view the CAN bus:

  1. Take a RJ45 patch cable, and cut it.
  2. Strip the wires and connect them to 8 position terminal block strip. The terminal block keeps the wires apart and organized, and lets you easily connect a terminating resistor. Some terminal blocks have built-in test points.
  3. I like to add a 120-Ohm resistor and use the breakout plug instead of a RJ45 terminator.

I have more on troubleshooting here?.

TIA T-568B Color Scheme

Pin NumberColorCANOpen Signal
2Orange CAN_L
3Green/White CAN_GND
4Blue Reserved
5Blue/White Reserved
6Green (CAN_SHLD)
7Brown/White (CAN_GND)
8Brown (CAN_V+)
RJ45 Plug Breakout with 120 Ohm Terminating Resistor

Last modified 4 years ago Last modified on Jun 20, 2013 6:00:09 PM