Analog Input Raw Value Fluctuation in S7200

[Nil 0]

Hi, The following problem have been noticed in recent projects which we have executed using S7200 PLC. We have been using 226 CPU along with EM235(4AI/1AO) and EM231(4AI) for quite a number of projects. What we have noticed during is that, say we have given constant 16mA to one of the Analog Input channel of EM235. In this case the Raw Count should be constant at 25600. But very frequently this number changes by + or - 1500 to 2000 counts. We also changed the 4 to 20mA souce & this is not working even the source is set to some constant value. The same is noticed even with EM231 Analog Input Cards. We tried changing the Analog Filter settings. But this is not working. Need some help............. Regards Nil

[panic mode 246]

this could be caused by different things. first thing to do is identify if the error in reading is due to faulty card or maybe program (possible but not likely) or due to external wiring (more likely). first thing to do is check raw values instead of processed and scaled result (you already do). you can bring stable signal source and confirm that card readings is stable etc. Connect multimeter at the same time (in series for mA, in parallel for volts) and check if reading make sense. there are couple places in wiring that are common pitfalls: - analog signal cables are not shielded twisted pair - make sure each channel is configured correctly (some cards require use of dip switches or external resistors when used to measure current). - cables are not routed away from sources of noise (like switching power supplies, VFD or servo drives etc.) - circuits are not isolated properly or are powered from not regulated power supply - commons are not wired correctly (should be short and all of them have to go to same point instead of cascading or daisy chaining etc.). if you have bunch of PLC I/O in one cabinet and filed devices such as sensors, indicators and valves in another (including analog inputs), do not share common for analogs (use separate cable etc.). - check for loose connections (pull wires one by one to verify if connection is good, check any jumpers, specially ones on terminal blocks such as side or center jumpers or bridges etc.).