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Old January 24th, 2023, 04:53 PM   #1
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justinfromPA is offline
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Cool Devicenet Module not communicating Data[0] bits

Hello, I am at customer site. The PLC is on and well, ie. no issues. The customer had to resplice a Devicenet cable not too long ago and the machine has not been working properly since. Right now, its set to communicate with a 1769-SDN/B AB PLC. I can see bits and high and low for Local 1: I.Data[1] through Local 1: I.Data[10]. I am not receiving any bits on the Data[0]. This is preventing the machine from running, since it doesn't recognize some Estops are good. I confirmed on the input card we see 120 VAC And the LED that indicates the input should be high is correct. All of the other Data bits seem to be fine, its just Data[0] that is not showing high properly. I am going to inspect the spliced cable soon, however im unsure how device net works exactly and if a bad cable somewhere in the network could impose problems somewhere else in the network. Any inisight on this would be appreciated, the machine is in a freezer
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Old January 24th, 2023, 05:26 PM   #2
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plvlce is offline
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It would be helpful to know if there are any errors coming up on the SDN card, and also what the layout of the network is.

The 1769-SDN card contains input mapping to move incoming data into the Local Data tag, but where in the tag that data goes is up to whoever configured the network. Data[0] is not necessarily coming from the same device as [1].

The most likely explanation imo is that you have a connection problem or other error with whatever node is mapped to Data[0]. The display on the front of the SDN card should indicate a node # and an error code, with the node # matching the device you're not seeing data from; look up the error code for a starting point on troubleshooting that.

Of course given this started just after a cable issue, looking there's not a bad place to start either. A bad cable can certainly cause issues elsewhere in the network, though I'd expect issues to generally be further along that section of the network since it's not taking the whole network down.

For troubleshooting cable, by convention the red wire is +24V and black is 0V. Blue and White are the data wires, and with power off you should read ~60 ohms across them -- each end of the network has a 121 ohm resistor.
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Old January 25th, 2023, 12:06 PM   #3

widelto is offline
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Justin: I got a program to diagnose Dnet thru CLX or Compact logix on line, just contact me I can give you a copy.
My two cents.
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Old January 25th, 2023, 02:04 PM   #4
Ken Roach
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A bit of exposition:

The 1769-SDN module works with both the MicroLogix 1500 and any 1769-series CompactLogix that runs the 1769 Compact I/O backplane.

On a MicroLogix, the first Input and Output word would be I:1.0 or O:1.0 and they would be dedicated to status and command bits. Mapped I/O from the actual field devices would start at the second word.

But this has to be a CompactLogix application since the I/O tags are in the "Local:1:Input.Data[x]" format.

In CompactLogix, the Status and Command structures are logically separate, and the whole Input.Data[x] and Output.Data[x] tag arrays are available for data from the 1769-SDN's Scanlist.

It makes some intuitive sense that the network designer connected safety feedback devices that map to the first parts of the data table; maybe they're also the lowest-numbered DeviceNet node. They don't have to be, but often people use Auto-Map or just by habit connect the first thing they design on the network with the lowest node number and the beginning of the data table.

I agree that the most likely problem is that the device that the e-stop feedback is wired to has been disconnected from the network, or was replaced and not properly reconfigured. Any error codes being flashed on the 2-character display of the module will be very useful information.
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