Devicenet dropouts

[Ken Roach]

My most notorious DeviceNet-related diagnostic project was on a conveyor system as well.

I showed up with my oscilloscope, my NetMeter, my protocol analyzer, and a folder full of firmware revision notes about the affected devices. This was a big deal; the story that "DeviceNet is no good" had been going around this major customer, all beginning with this one conveyor.

The lead electrician showed me the conveyor. He steadied his hand on the frame while pointing out the upstream and downstream sections.

After about ten seconds, he jerked his hand back. "Ow! F*** !".

He knelt down to show me the VFD drive enclosure, and opened the door to it. Another ten seconds went by. "Ow ! F*** !".

We walked under the conveyor to view where the cables ran to some DeviceNet photoeyes. He reached out to the frame, then paused, and grabbed a structural I-beam instead.

At this point I had to ask: "What, um, just happened there ?"

"This ***ing conveyor always shocks me."

A millwright chimed in. "Yeah, that one gets me every time too. I hate working on this conveyor. You've got to fix the network".

I put my hand on the frame. "Ow ! Mother****er !"

They had recently changed conveyor belt vendors, and the new ones had nylon lacing instead of steel, so they didn't discharge their static every time the laces passed the steel tail pulley. In addition, a new pedestrian walkway had been installed, and the welders cut through the grounding cable that had previously tied the conveyor to the building's steel frame.

So the only routes from the conveyor to ground for the highly accumulated static charges were through the flexible conduit containing the power cable, and through the grounded DeviceNet cable shield. Or, if you happened to be holding on to the machine, through the body of an unlucky electrician.

The network itself was actually handling the sparks well; the VFD's daughtercard was rebooting when it took a 5 kilovolt spike through the ground plane, but the scanner and the photoeyes and the rest of the devices on the network rode through with no ill effects.

 

[ASF]

What, devicenet can't handle a 5kV spike without dropping out? What sort of useless, no-good network is that?

 

[proof]

Devicenet, like all digital networks must follow the rules for installation. I've had networks (not just devicenet) that gave intermittent problems because the drop lines were too long, and especially the drops at the end of the main trunk line. If long drops of similar lengths are used at the end of the main trunk, the end of the trunk looks like the end of a snakes tongue. For example, If the last two drops were 20 feet long, I would extend the main to go down to the 2nd to last device and back then over to the last device. So instead of having the trunk ending with a split of ~20 feet, it ends with ~ 60 feet of trunk line.

 

[diat150]

I am back on this issue. One thing I noticed is that the Can H and Can L voltages are higher than normal according to the troubleshooting guide.

With a network master connected and
polling the network, the CAN-H to V- voltage
will be around +3.2 VDC. The CAN-L to V- voltage will be around 2.4 VDC.

I am getting around 4.3 on can H and 3.1 on Can L at the devicenet scanner. At the nodes, I get 3.6 and 2.4. On a separate Devicenet network that is working properly, but is not grounded or on a separate supply, I am seeing 3.6 and 2.3 at the scanner, which match up with the troubleshooting guide.

I have already got the devicenet on a separate supply which is set to 24.1 volts. I have also corrected the grounding issues according to the troubleshooting guide.

Any idea what could be causing the higher voltages at the scanner? The troubleshooting guide doesnt say anything about high voltage causing a problem, it only suggests that low voltage would be caused by a bad node.

 

[Gil47]

I was going to say the terminating resistors are missing, but you say you replaced them,
so my wild guess now is make sure the terminating resistor is connected
to the correct lines and not across the power lines.

What sort of DN cable is used, and what sort of Taps are you using.