Of Terminators and Characteristic Impedance

“Terrible what’s going on in this country,” Dan said, watching as I worked, tracing the data bus wires. “We’re proud of our heritage and now folks are taking it away from us,” he said as I worked. His truck had a DTC 148309, kept going into regeneration and he had it in to three different shops that replaced the complete aftertreatment system, including the doser valve three times.

“What do you think about all this stuff going on, Bob?”

“Actually, Dan, your truck is currently consuming whatever brain power I have at the moment,” I said.

He nodded. “I asked the technician working on it the last time to explain the J1939 data bus, but he wasn’t very clear.”

“The J1939 multiplexed chassis uses twisted-pair wiring, twisted to prevent the data backbone from acting as a radio antenna. The wires twist a through a full cycle once per centimeter on the backbone—2-1/2 times per inch,” I said.

My hand was getting cut up from the tight places I was going into.

“Those terminating resistors—they’re for signal noise, right?” Dan asked. “And the technician said they need to be 120 ohms, right?”

I checked V-CAN, E-CAN, D-CAN and A-CAN terminating resistance. The resistors each came in at 120-ohms.

“The truck J1939 data bus has a characteristic impedance of 120-ohms, but don’t confuse this with terminating resistors used at the end of the data backbone, Dan.”

“What is characteristic impedance, Bob?”

“It is just a factor of how a conductor appears to a high-frequency signal, regardless of length. You can’t measure it with a meter.”

Andrea, Dan’s wife, brought us some homemade lemonade. I loved her deep-southern accent. “Y’all thirsty?”

I thanked her and she noticed my bloody hand. “Y’all get into a cat fight?”

“No, just Dan’s truck,” I said.

Andrea offered to take care of the scrapes but I refused, saying that I was planning on getting a few more. She shook her head and went back towards the house.

“The terminating resistors also provide a low-resistive pathway for current flow between CAN H and CAN L. The capacitance can discharge and cancel rapidly,” I explained. “Of course, the length of time for a capacitor to drain is proportional to the resistance of the conductor it is discharging through. If capacitance can’t rapidly discharge when a component is trying to transmit at a low level, then the voltage differential between CAN H and CAN L stays high and that can shut down the data bus.”

“The truck has been stalling at times,” Dan said. “I had to disconnect the battery a couple of times to get it started.”

I scraped my hand up again, getting at the hidden wires while Dan held the light. “How do they come up with this impedance?” He asked.

“They do it by maintaining the dielectric constant, or wire insulation, and keeping the actual physical space between the two conductive wires constant by having a consistent twist pattern. Twist pattern is very important,” I said.

Then I found it. “Here is the problem.” I said. Someone repaired the CAN bus.”

“I could have told you that,” Dan said, “The dealer technician did it. He found some rubbed-through wire.”

“Yeah, but substituting ordinary wire for J1939 cables is a no-no because the dielectric properties of the insulation should be constant. You can corrupt communications on the bus by altering the impedance.”

“Corrupt? How, Bob?”

“Because the amount of time needed for every signal pulse pumped down the bus to neutralize increases; now the reflected signals scramble the data bus,” I said. I repaired the wiring, fired up the rig, and checked the data bus and all was well.

“I think we have it licked, Dan,” I said. Everything looked good.

We had great conversation later on over lemonade on the porch while Andrea insisted of doctoring up my hand. We talked about the protests and expressed feelings, but the world is tainted enough without letting it spill into my blog or Facebook.

Dan and Andrea are great people. I wish the world had more like them.