DH+ intermittent problem


Thread Starter

Jett, Darin

This is one of those small problems that I've been living with for about a year, trying different things to isolate/correct it periodically with no success. Now I've run out of ideas.

On one DH+ network with 7 SLC's and 4 computers with KTX cards, I have one node that the logging computers miss data from a couple of times a day.
Data is logged every hour for one application and every 1/2 hour for the other application. Normally data is missed 2-5 hours per day on the hourly and double that for the 1/2 hour app. The logging is done with FoxPro through WinLinx DDE.

The other apps rarely miss any data from other stations on the network, the diagnostics in RSLinx on my desktop show 0 errors over the course of a week or so, the messages from one SLC to the other and back run fine, I have no problems when working online with this node in RSLogix, in essence, everything looks great but we miss data.

The network is DH+ in a daisy chain configuration, about 800 feet long (max)
with terminating resistors on both ends and running at 56K. Just yesterday I changed the processor out for a brand new one and installed a UPS on the power to the rack. Last year I had a new ground driven and hooked up properly. This node is also the closest of all the SLC's to the logging computers.

Could the rack power supply be causing problems? That is one thing I haven't changed so far.

Any help is appreciated.
Darin Jett


Hi Darin:

Quite often, data goes missing when grounding systems aren't installed perfectly. If you want some ideas contact me directly and I will try to

Bob Pawley

C. Thomas Wiesen

The problem might be the DDE link between Winlinx and your data loging application. I set up a simple data collection routine using an NT laptop
running RSLinx and a DDE link to an Excel Spreadsheet. The laptop was connected to a AB SLC via PC card and DH+. The DH+ network had a number of nodes on the network, ~16. Several words of data were collected every 5 seconds. If I was using another application on the laptop, I would occasionally miss a data point. I never pinned down the precise problem, but I never missed a data point when the laptop wasn't busy with another application. You might want to look at how your DDE link is set up and whether you could use a different method to collect the data (OPC?).

Robert McDonald


I would suggest starting to look at the FoxPro application. A few questions would need to be answered.

1. How do you initiate the logging ?Individually for each node or all with one trigger?

2. What exactly do you use to trigger the logging? time or change event?

3. Is there any correlation between PLC unit numbers and the lost logging?

4. What naming convention have you used with the DDE interface between Winlinx and FoxPro for each PLC's data tags.

Some or all of the above would help pinpoint the problem.

My gut feeling (for what that's worth) is the problem will be software not hardware.

Robert McDonald, Managing Director
Email: [email protected]

RM Automation Pty Ltd
"Computer Automation Engineers"
Phone: 61 8 8346 7733
Fax: 61 8 8346 7929
Hi Darin,

1. Could there be a problem with the FoxPro application? Have you put a communications analyser on the DH+ and watched all
communications from the suspect SLC to determine if data is available to the FoxPro but perhaps not being recorded? You could install another node on the network with different hardware and PC software to log all data with the suspect SLC's address. This record could then be compared with the FoxPro record.

2. Have you swapped the suspect SLC with one of the other SLCs or a new SLC?

3. Have you swapped the KTX card in the FoxPro computer, replaced the FoxPro computer and re-loaded the FoxPro logging application?

4. Have you tried changing the DH+ logical address of the SLC that is missing the data (nearest SLC)?

5. Have you examined the DH+ network and cable:
a) Using an oscilloscope to check DH+ signal levels at various points etc.
b) Have you metered the DH+ cable for shorts / low or high resistance.

6. Have you replaced the DH+ cable to the (nearest) SLC?

7. Have you replaced the DH+ cable to the FoxPro computer?

8. Have you tried adding a termination resistor to the suspect SLC?

Hope this helps.


Aly Mawji
DataLink Technologies, Inc.
(800) 851-7883 x 23