DH+ data highway traffic congestion


Thread Starter


Hi, I have an plc network that gives
slow responses to reads and writes from the
host computers. It consists of 4 AB plc5's and 3 AB slc 504's, the two hosts are Factorylink 6.6 with KT cards mounted in them and connected on the DH+ data highway via RS Linx.
Hope to be able to find some software or even documentation to help me determine if the data highway is at saturation.

Thanks in advance.

Jeremy Pollard

I can already tell you it is - any HMI will make requests almost faster then the network can handle it. And being a token passing network serial - well you just about beat.

Did a test with SLC 5/03s and panelviews - token time went up over 10x when panelviews were added. Huge overhead. An easy test is to put a msg instruction in each PLC and a timer tied to the done bit and enable bit. The time will tell you howlong it has taken the msg to initiate and to complete in each PLC - I would read 10 words from one PLC.

Do this without the Factory link running if possible. Then repeat with one station then the second one. You will see the difference. The HMI s/ware will maximize the number of requests per token which basically takes up all the bandwidth.

Cheers from: Jeremy Pollard, CET The Caring Canuckian!

Automation Linse

Some things to consider:

1) You could consolidate data within the PLC. Yes, it is "nice" to have the HMI fetch the actual data scattered throughout the real program. However you could create a new INT/N file JUST for HMI data and modify the ladder-logic to copy/concentrate the HMI's data into the new file. Reading 50 words from 1 file will be 10 times faster than reading ten blocks of 5 words each from 10 different file areas.

2) Consider using a Rockwell ENI or Digi One IAP on the RS-232 port of some of the larger PLC. This would move the HMI traffic *OFF* the DH+ and allow it to run in parallel via Ethernet and the RS-232 port. I have many customer doing this; some to increase HMI performance, and some to be assured the HMI has little impact in PLC-to-PLC peer interlocking taking place on the DH+ (or ModbusPlus) network.

best regards
- LynnL, www.digi.com (Experts in Ethernet-to-Serial)

Maurice Pinner

You have had 2 good replies already, but here is some more help.

Firstly, if you are using new type PLC 5,s they can present 2 data highway nodes. It may be possible to create a second DH+ for the stuff you need to work quickly.
Secondly, many highways get jammed with unnecessary data calls. You should take the advice given and consolidate your data calls, i.e. copy data in the program to a single file and read that file over the DH+. Each data call has big overheads, so one big call is much better than many small ones.

The suggestion of writing a msg instruction to test saturation is fine. Also slow online response to the programming terminal is a good clue that the DH+ is overloaded.

Next check the DH+ node numbers allocated. Keeping the nodes 1,2,3,4,5 etc. is better than 10,20,30,40
Lastly check out the physical cabling. Make sure terminaters are correct, the cable is kept away from electrical noise, and if you use drops, that they are at least 10m apart on the highway. Ant bad cabling practice may load the highway with bad packets. Use RSLinx or WINLinx utility to look at the transmission errors.