PROFIBUS Issues and SDAT Matching

I had a call from a customer, working with some client supplied equipment. In difficult economic times it is natural for customers to do what they can to reduce costs. Using existing inventory or sourcing used equipment is a common occurrence.

Our customer mentioned they were not able get the 83F’s to communicate to their PLC on a PROFIBUS network.

Upon arrival I quickly hooked up our ProfiTrace Ultra Pro.

I was quickly able to observe issues happening on the network.

Voltage Issues

Reflections

I checked to see if the termination was on at the PLC, and it was off. After turning it on the voltages looked much better and the waveform improved significantly. The meters were now able to enter into a data exchange with the PLC, save one.

I observed that they had some cabling issues. Wiring between a few of the meter was less than the 1m cable required. There was also no active termination at the end of the network. Active termination reduces the reflections by pulling up or down voltage. PROCENTEC has a nice active terminator called the T1. I like that it requires no connector (included) and a spot to plug in to the network with a PROFITRACE.

While termination can be turned on in a 83F, it is not easy to access and if the transmitter is pulled out of service, I would guarantee the person replacing would not turn it on in the new transmitter, not disable it in the old, and if they put it back into service would have a very difficult time identifying that issue.

Old Problem, New People

Observing the meter that would not provide data, we noted it was in an error state, displaying error #380.

My first instinct was to think that the sensor was faulty. A quick resistance check noted something odd, 20 Ohm on the drive coil, while not outside the realm of possibility, meters with this resistance are not common.  I checked the other meters and they were all ~100 Ohm. This again led me to believe that perhaps the sensor was faulty. I ran a Fieldcheck to test the sensor. It passed. Once I was able to download the report and review, I noticed something odd. The report said that it was a ½” sensor. I was sure that it was a 1” sensor that I had observed.

Going back to site, I checked the Serial Number on the sensor (as listed on the sensor label), and it was different than the one indicated on the report. I checked the rest of the 83Fs, and found 3 others that didn’t matched. I tried a 1” SDAT (sensor data chip) in the meter that was reporting the error. The error cleared and the meter entered data exchange and providing measured values. I advised the client to match the correct sensors and SDATs. This resolved their issue and other issues that they had not yet identified.

As much as we would like to think that we can manufacture the exact same thing, there will be a variance. Endress + Hauser helps to minimize this issue through their QC and QA, but every sensor is going to be a little different, and with a high precision instrument you deal with those differences with an SDAT. While you can use an SDAT from a similar sized sensor, it will appear to be working just fine, and it will likely be close, but there is a high probability that it will not be within the accuracy statement of the meter. Many of our customers will simply swap out equipment if there is an issue, and SDAT’s can get lost, left in the transmitter etc. With the next generation Proline, we can reduce this issue on instruments with remote transmitters. You can select to have the SDAT installed at the sensor. This is a much better option as technicians no longer need to open the transmitter, and no further steps are required. Additional features such as the webserver and Heartbeat allow for remote troubleshooting capability, helping to save time and money.