A-B DH485 Networking


Thread Starter


Does anybody know of an A-B DH485 network with more than a dozen nodes? (that doesn't have comms problems). Where i work we have a network of 14 nodes and we have problems accessing the network using an AIC unit with rslinx. (A-B claim that a DH485 network is good for 31 nodes). The nodes include 8 slc plcs, 5 Panelviews & a KA5 bridge card to a DH+ network. We also have a KT card in the pc which allows us onto the DH+ network (consisting of 3 plc5's & a citect pc) which works fine. If we access the DH+ network, then go to the bridge card, then we can access the DH485 network ok (but not via the AIC unit!!). The panelviews talk ok to the plcs, & vice versa. Problem first arose when the last two nodes were added. Any comments much appreciated.


Frank Mitchell

We have four DH485 networks each having sixteen nodes not counting a computer and a bridge node. There are eight SLC503's and eight Panelview 550's and a bridge node of a SLC504. When you push a button on the Panelview it is a little slow but they seem to work ok. At first I tried to make the node number scheme match and so I had node numbers up to 28 but then to speed it up I relented and used the lower node numbers 1-8 and then made the max node addr = to 18 but that did not speed up the network very much. We send and read 30 words to from each node from a central SLC504 in a master slave configuration. We poll each node from the central 504 about every half second. So it takes about 5 or 6 secs to update all SLC 503's. If I go any faster I start to get comm errors returned to the central SLC 504's Message instructions. I have not tried SLC5's in multple configurations but would expect performance to be better.


Usually A-B documents one thing and reworld may be something else. I have heard different A-B people claim that 10-12 nodes is all the throughput you will get with DH-485. We are an integrator and we have seen major problems with networks as small as 3-4 nodes. There are multiple factors including cable quality, connection quality, length of cable runs, stable power supplies and the list goes on. You have probably reached a limit without going to a more robust network. Just my opinion....

Jeremy Pollard

Comms degrade after 10 nodes. Did timing tests and to send a word of data
from 3 PLCs with 3 PV attahced was awful to say the least. Add a
programming station and your done. I'm talking 10 secs..... although this
was 2 years ago.
Even DH+ limits out with any level of traffic

So 31 nodes is addressable, but not practical. All about the peer to peer
stuff. Try maxing out the station addresses to the max on the trunk, but
wouldnt expect any difference.

Cheers from:

Jeremy Pollard, CET
[email protected]
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Also known as the Data Sidewalk, the DH485 in our press factory has 19 nodes with two PC's, having a KF3 module and a KTX card respectively. We used to have problems with it until we checked the cable and all connections and found both wrong. Since we repaired as per the AB manuals direct, we have been okay.

Rockwell suggest DH485 is only good for 16 nodes in practise and only then for basic traffic amounts. I would suggest you check your cabling/connections first, traffic second, if you still want to stick with it.

Good luck

Mike Lynch
Canpac International Ltd
What is the distance between the first and the last node?
What type of cable are you using i.e. Belden xxxx.
(Resistance & capacitance spec.)
Is your bus terminated with a resistance at both ends?
Is your cable shield terminate at one end of the bus (at least at one point)?
Is your cable shield continuous on the entire bus i.e. no brake at one node?
Do you have a quality ground at your cable shield termination?
Are your cable pairs consistent from one end of the bus to the other?

Just asking?


many thanks to all that replied,
I can now approach management with some justification for a more 'robust' network (probably ethernet)

Kinner, Russ

Recently I was assigned to a client's maintenance project that had 6 Intelution terminals and 8 PLC-5s on a single DH+. Update times when we connected a programming terminal exceeded 20 seconds. DH485 overload is not unique. I've always had difficulties with performance on lower speed (under 100 KBaud) PLC networks having more than 7-8 nodes.

We migrated 4 of the terminals to Ethernet after replacing the PLC-5/60s with E-Net versions and adding a second E-Net card to the Intelution PCs.
It still makes for a long update time on the remaining DH+ only PLCs, but we don't get kicked off if we try to edit a program as we did before. Its pricey to replace several processors and add CAT5 cable around your facility, but the 200x improvement in raw baud rate between the units may be worth it.

In your case, if you replace the SLCs with DH+ capable versions and use DH+ to separate some data away from the DH485 wire, the performance may be improved enough to help. Possibly switch the HMIs to DH+ and let the data flow there. A change like that may provide just enough improvement in performance to satisfy your needs. The HMIs almost always hog any communication cable.

Russ Kinner
AVCA Corporation
Maumee, OH USA
*Very hard to determine cable lenght as it is spread throughout the plant, however after I first posted this question I obtained an A-B manual for the 1761 AIC units that we use & effective network length apparently can be doubled (to 8000 feet). I am yet to try this.
*Sorry don't know cable specs. Cable cores are different colours (ie red,white, black & green...)
*Have double checked terminations at end of nodes & appear ok(DH485 doesn't use resistors, unlike DH+, according to the info I have been given)

In your case, you don't need separate terminating resistors at the two ends of the DH-485 network, but you do need to jumper terminals 5 and 6 on your AIC to enable the built-in 120 ohm terminating resistors.

You can get a good online reference for the AIC at
http://www.ab.com/support/ by choosing "Cables and Communications" under the
"Networks and Communications Devices" heading. The first document listed is for the 1761-NET-AIC.


Mark Wells
Runfactory Systems Inc.
http://www.runfactory.com1235 Bay Street, Suite 400
Toronto, Ontario, Canada M5R 3K4
Phone 416-934-5038
Fax. 416-352-5206