Frequentry CNF

[Koni]

We are facing frequentry CNF(Connection Failure) between DCS and some field devices.PLease give me you advice.
We use Yokogawa CS3000R as DCS. This CNF comes from a FF field transmitter and a FF smart positioner below.
Foxboro Eckardt Model 240FF (Transmitter) 5pcs
Siemens Model SIPART PS2FF (Positioner) 2pcs
CNF is cleared when we reset device power. However it will be resumed after 1 month around. We try to trouble shoot it as below, but not yet cleared.

CNF problem is not segment wise. these are scattered on several segments.
Spar cable length of each is less than 100m, total length of all spar is less than 800.
Signal/Noise level of FF communication cable is good.
Grounding of spar cable and trunk cable is good.

I apreciate it if you could give me a effective advice to clear this trouble.

Koni

[aagostin]

Can you post the readings that were the basis for you saying:
"Signal/Noise level of FF communication cable is good. Grounding of spar cable and trunk cable is good."
Was this done with FBT-6? If not, it would probably be wise to use FBT-6 and checkout the respective segments, then post the readings taken with this tool (you can transfer the data to Excel and then post the XLS file).

There can be various issues; most issues are undoubtedly installation related; these are things like loose wire, shorts to (local) ground, water inside device, etc etc.

Without a dedicated fieldbus tester, you may not pick up the relevant information, such as retransmissions, noise in the different frequency bands, etc.

The last option you should look at are software issues. There were some incompatibilities reported in the past, bust these are getting very rare nowadays. Also, device failures can not be excluded, but are extremely rare as well.

So concentrate on the installation issues, and you have a high chance to find the problem.

[CHRISC1024]

There are A TON of variables that can cause this type of failure. Do you have an FBT-6 or other Fieldbus tester? If so, can you post a report so we can see what the signal levels look like? How about a scope meter? This can also show noise and AC voaltage induction and can be used to verify proper grounding. Is it the same device on each segment that is failing? It can be a loose connection or if it is coming off of a multiport tap, a bad port on the tap. Also, it can be another device that is having an issue that can drag down another device.

If you have a loop sheet or drawing with the segment layour and some basic readings, it can help narrow down your issue. I have delt with these before and 80% of the time it's something simple but that other 20% can be a real headache to track down.

[jlash]

I have run into this issue once before.. there was corrosion on the device terminal strips and the device was only seeing around 6v.. But as Chris said... this could be caused by anyone of many factors...

[marisg]

The biggest cause of intermittent network operaton is loose screw terminals. Try using a torque-limiting screwdriver such as WIHA 291 04 handle with blade 285 37 to tighten all the connections. Another problem is cable wires that are not tin plated.

[ccarter]

Over 500 students (credit students, industrial clients including engineers, techs, installers, supervisors)
Regular lab exercises involving building segments, testing, removing and reinstalling devices, etc.
Terminal connection failure rate without torquing screwdriver = greater than 25% (conservative)
Terminal connection failure rate with torquing screwdriver = less than 1 %.
Torquing screwdrivers - Don't open your toolbox without one.

Then some jerk walked away with the two that I had, so now I have to wait for a few months to get them replaced (college budgets and all).

Chuck Carter