pelly09

DH+ issue

8 posts in this topic

We have an SLC5/04 which is connected to a PV1000 via DH+ and all is working well. My boss wants me to add the PLC to our existing DH+ network which I believe is trunk and dropline. I have a station connector in place on the network (trunk) and have run in a new "blue hose" to the PLC (less than 100ft). However, when I connect the new cable to the PLC the PV shows the message "error 688, no network nodes can be found". I have removed the terminating resistor from the SLC and left the resistor connected to the PV1000, I believe that the baud rate for all devices are set to 57.6K. Is it OK to "daisy chain" devices on a dropline as I have never seen this before?

Any guidance would be most appreciated.

Steve

Edited by pelly09

Share this post


Link to post
Share on other sites

So you have (2) DH+ networks, 1st being the local SLC-PV1000 network and 2nd being the plant DH+ network.  One cannot simply connect the 2 together and expect harmony as DH+ is an addressed node arrangement.  Typically, local DH+ networks, such as your SLC-PV1000, would be PLC = Node 0 and HMI = Node 1.   One cannot have two or more devices on a DH+ network with the same node address.

First, put the (2) DH+ networks back as they were and get them back up and running properly.

Second, look for a DH+ network drawing (if it exists).  If you cannot find it, then you will want to start mapping out your network.

Third, read up on DH+ so that you know the rules.  Terminating resistors at both ends, baud rate options, how many drop nodes you can have, length of drop lines, length of trunk lines.  When you touch a working DH+ network to simply add another node, do not be surprised when the rest of the DH+ network exhibits strange behavior.  The strange behavior can easily be unplanned production.

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1770-um022_-en-p.pdf

From my DH+ Primer:

-          DH+ is a 57.6K or 115.2K token-passing peer-to-peer network for less than 20 devices

o   DH+ was designed for trunk lines and drop lines

§  Trunk lines run the length of a network and terminate into drop line drops (1770-SC station connector)

§  Drop lines run from station connectors to the device

Share this post


Link to post
Share on other sites

kaiser brings up many good points.  The most important is your network addressing.  I have found over the years that DH+ is pretty forgiving and have seen some pretty poorly designed/installed systems that run without issues.  Four things you have to be absolutely certain about:  you cant add duplicate addresses to the network; your wiring has to be consistent (clear, shield, blue) and proper calble (blue hose - belden) is important;  Your baud rate has to be the same (i remember 3 - 56k, 115k, and 230k but maybe that last is just RIO); the terminating resistors must be installed and correct.  They resistance is dependent on the network speed.  Total network length can be an issue but not generally as at 56k you can run 10,000 feet.  Trunk and drop are how DH+ was designed; however, I have seen maybe 2 networks installed that way over the last 25+ years.  Generally you will find that DH+ is daisy chained from one device to another.  Not saying to change now that your network is installed to spec; however, I am doubtful that how you connected the components is the issue. 

Share this post


Link to post
Share on other sites

Thanks for the replies guys!!

I'm pretty sure, but not certain, that I haven't duplicated a node address (I will be checking this today). I am pretty certain, as I have checked other devices, that the baud rate is 57.6k. And I am pretty certain that the wiring is connected correctly (that was checked last week).

I have disconnected the PV and just hung the PLC on the network but it doesn't show up in RSLinx. I will also be trying to find an original drawing of the network to confirm that it is of the trunk/dropline variety and not just daisy-chained to each device. Just as a side note, if I discover that I have a daisy-chained system, I assume that connecting a PV into the same port as the network will cause me grief? The station connector that I have connected my dropline into is where a machine used to live, this was removed before I joined the company (station connector may have replaced the PLC in the daisy-chain) so until I find a network drawing I cannot be 100% certain that it was connected via dropline or daisy-chained.

Steve

Share this post


Link to post
Share on other sites

Sorry I should have been more clear.  If you have a station connector, then you have a trunk/drop structure.  When people daisy chain DH+, they do not use station connectors.

Share this post


Link to post
Share on other sites

Where we have a station connector is where a machine used to live, I was unsure whether the dropline had been removed when the machine was taken out or whether the station connector had been put in to replace the missing PLC. As it turns out we have a trunk/dropline system.

I checked everything that has been mentioned, node numbers, baud rate, drop lengths etc. and everything was alright. This is an old network that has been added to and modified many times over the years and we do not have any drawings/documentation for it. So, I decided that I would walk the network and identify all the nodes/cables and PLC comms channels that the cables were connected to that I could. While doing this I noticed that on a number of the PLC's the DH cable was connected opposite to the others with the blue and clear/white wires connected to pins 1 and 3 on the 3 pin connector reversed (some connectors had blue to pin 1 and some had clear/white to pin 1 and vice versa). After seeing this I thought that I would try reversing the wiring to my additional PLC and to my surprise it worked!!

PV is working correctly and both PLC and PV are visible in RSLinx.

I think further investigation into the network is required to understand how it is put together.

Thanks to those who offered help and advice, it is much appreciated.

Steve

Share this post


Link to post
Share on other sites

DH+ is a twin-axial cable.  It is not unheard of for the system "to work" with the "blue hose" (typical Belden DH+ cable) not terminated properly.  Then a small change to the DH+ network and intermittent operation occurs.  

I like to develop Primers or Work Instructions and make sure the technicians have access to (and utilize) the information.  Lessons Learned go at the top.

Share this post


Link to post
Share on other sites

Yes the expected wiring is Clear-Shield-Blue with DH+, but as long as it is wired incorrectly on both sides it will still work.  That would be worth fixing over time.  The whole reason for the wire sequence is to differentiate it with Remote I/O which is supposed to be wired Blue-Shield-Clear.  Maybe you can get all of that worked out when you have some downtime.  It is not a good idea to be working with the network at all while the rest of your plant depends on it.  That's a good way to get some unexpected downtime. 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now