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Anticipating a faster bus

TAMPA, Fla. – The SAE J1939 data bus is about to get a whole lot faster. And that, according to Ken DeGrant, field applications engineering manager with Dearborn Group Technology, is a development that should be welcomed by technicians.


TAMPA, Fla. – The SAE J1939 data bus is about to get a whole lot faster. And that, according to Ken DeGrant, field applications engineering manager with Dearborn Group Technology, is a development that should be welcomed by technicians.

DeGrant provided a preview of upcoming changes to the J1939 data bus during a session at this year’s Technology & Maintenance Council meetings.

He recalled the anxiety, worry and misinformation that surrounded the industry’s shift from a J1708 data bus to the J1939 standard that’s been in use for about 10 years now and looked to settle any such nervousness as the industry prepares for its next significant change.

DeGrant said the new standard is still two to four years away, and should not be feared by truck technicians.

Double the speed
When the new standard is adopted, it will effectively double the J1939’s speed from 250 KBps (kilobytes per second) to 500.
Essentially, the transition will double the speed of the data bus, enabling it to better handle the strain placed on it by today’s telematics devices and tomorrow’s emissions requirements.

Today, said DeGrant, fleets are averaging 35-40% bus utilization. When that rises to about 70%, he said “you are really starting to push the envelope and you’re going to start dropping messages and applications are not going to work as they should; things are going to start happening.”

To avoid that scenario, OEMs are expected to move to a faster J1939 data bus within the next few years.

The 500 KB speed is already standard in the automotive industry and DeGrant said it’s ideal for the trucking industry as well, since going a step further to a 1 MB baud rate would increase the likelihood of wiring problems related to water intrusions and scuffing of the vehicle frame.

“I’d say the J1939 500 KB is going to get us another 15 to 20 years,” DeGrant predicted.

The alternative to increasing the speed of the J1939 data bus would be adding a second bus, which OEMs could still choose to do, DeGrant said.

In that event, the OEs would provide two buses to handle separate streams of data: essential data such as that from the engine and transmission and less important data, such as that from bodies and telematics devices. The advantage of moving to a single J1939 500 KB is that “it is not changing the way we transmit data today,” DeGrant noted.

Provided OEMs move to a J1939 500 KB data bus, wiring will remain basically the same as it is today, DeGrant said.

The upgrade will allow a longer maximum stub length of nearly two metres with a maximum distance between the farthest nodes remaining 40 metres.

The diagnostic connector will change: there’ll be a nine-pin Type 1 connector which remains the same as today’s standard while a Type 2 connector will also be offered for 500 kB applications (the only difference will be the diameter of one of the pin holes). Type 2 connectors as well as the 500 KB cables will be coloured green to avoid confusion.

To prevent damage, a Type 1 cable will be unable to plug into a 500 KB data bus, DeGrant explained. However, the new cables will plug into either a Type 1 or Type 2 connector.

Technicians will be pleased to know they won’t have to upgrade their diagnostic adaptors. It’s not yet clear whether they’ll require new handheld scan tools, however.

Upgraded cables
“If you are using a handheld scan tool, ask the manufacturer if you get vehicles that are going to have the J1939 500 KB, because you want to make sure they’re capable of that,” he suggested.

Fleets will need to upgrade their cables for the diagnostic adaptors.

“There’s no way of getting around that,” DeGrant warned. And they’ll also have to update their OEM software applications.
Vehicle Diagnostic Adaptors (VDAs) will be capable of automatically detecting baud rate, so they’ll be able to plug into the data bus and determine whether it’s a 500 KB or 250 KB data bus without creating errors.

All things considered, DeGrant said the transition should provide minimal disruption for technicians.

“It’s actually pretty good news,” he said. “There are not a lot of things it is going to affect other than the diagnostic cables. Everything else is pretty much staying the same, it’s just creating more lanes on the highway to get information across.”


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