1 DH+ node drop causes all DH+ nodes to drop

bigLee

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Join Date
Jun 2018
Location
Florida
Posts
77
Hello,

I am currently having issues with 7 PLC5's daisy chained via 1771 FA fiber optic converter modules with DH+ blue hose off those to the controller.

If I pull one dh+ connection, 1 fiber connection, or even power off the PLC, I lose all DH+ network comms to PLCs.

I understand if it is in the middle of the daisy chain. I would lose that and all down stream controllers. But I am starting with the last node. Only one fiber in to the 1771 FA module and a blue hose from there to controller.

Can't make much sense of this. All controllers have resistors. My node 0 has a S10:1 dh+ data table changed minor recoverable fault.

I'm trying to migration all PLC5's to contrologix. Wanted to start with this one as it's on the end of the daisy chain. But it kills the plant if I do. Same with all other controllers on DH+. Power off, remove fiber leg, remove dh+ port. All scenarios crash the network comm.

Any ideas on what this could be? AB support seems stumped as well.

Much appreciated!!
 
Thanks for the detailed description !

If you disconnect the controller at the "end" of the network, can you recover DH+ token passing (a solid green LED on the DH+ port of the PLC-5) among the remaining PLCs by simply waiting, or by cycling power or disconnecting/reconnecting to any of the other 1771-AFs ?

If you reconnect the disconnected controller and 1771-AF, does the DH+ network recover by itself ? How long does it take ?

My recommendation is to check the "blue hose" twinaxial cables carefully for termination resistors and cable length.

You noted that "all controllers have resistors". That might not be sufficient to create a stable DH+ network.

All DH+ physical copper wire segments need to be at least 10 cable-feet in length and have termination resistors on *both* ends. For 57.6 kb/s DH+ those should be 150 ohms, and for 230.4 kb/s DH+ they should be 82 ohms.

This thread also reminded me that the "Classic" PLC-5/10-12-15-25 controllers have an internal termination resistor that can be switched on using a chassis DIP switch. Are your PLC-5's Classics or Enhanced (like the PLC-5/20 or 5/40) ?

The shield wire should be landed on the terminal strip on the 1771-AF, and in the center terminal on the DH+ 3-pin connector, but not additionally bonded to earth ground.

The 10-foot minimum sounds a little counter-intuitive, but it is a real thing.

Systems that use 1771-AFs need to be really robust before you start adding 1756-DHRIO's to them. Get all the error counters down to zero and clean and re-terminate every wire.

I'm having flashbacks to the last paper mill DH+ I analyzed. I had to trim back 3 or 4 inches of cable before I got to bright copper, and had a bucket of vinegar for refurbishing swingarms. Rust never sleeps, and neither does sulfur dioxide.
 
Ken,

Thanks fir the follow up! When reconnecting the last node back after comm crash, all comms return quite fast. I’ve really only given at max 3 minutes after disconnect to see if comm return. What is a standard time you’ve seen?

I did notice it will come back for a second then drop off again for the long duration. But I will give the disconnect/reconnect of still active nodes a shot.

The blue hose jumpers from FA module to dh on controller are no longer than 14”.
This install was probably done before I was born.
 
The robustness of DH+ and RIO makes bad and damaged wiring and topology work well most of the time, leading to a lot of complacency.

I once troubleshot a big DH+ network in a "pickling line"; aluminum, not dill. The customer had a stack of a dozen PLC-5/40 controllers they had already replaced, every time one "fell off the network". Easily $100K of hardware, before they ever called me.

I spent two graveyard shifts (the only time I could touch things) peeking into cabinets, tugging on cables, and carefully writing down diagnostic counter values.

Until I found a PC on a desk in the middle of the shop, with a 1784-PKTX card in it.

Balanced perfectly across the blue and white conductors, in the two millimeters between the connector and the insulation, was a staple.
 

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