Hello Experts,

I am in very strange situation and struggling to fix it. Following is the configuration:

1. S71200 CPU-1211C with CB1241 for modbus serial communication

2. Siemens SMART 7KT Energy Meter

3. TIA Portal V16

Please check the attached screen shots.

I am trying to get the data from energy meter and the communication is OK but KWH reading is not matching what it is showing on the energy meter itself. So I collected the data from the same energy meter using ModScan where the KWH reading is exactly the same as displayed on the energy meter.

Also notice the data acquired from the energy meter by PLC is correct till "FREQUENCY " but after the KWH reading is not correct. The Frequency is being read from input register 30057 and KWH is from 30059.

Please help. Thanks a lot.

For corporate reasons I cannot view the date because I can't download the zip file.

But Smiffy's explanation sounds like a rarely encountered Modbus anomaly when reading and interpreting a Floating point value that is in an adjacent register to another floating point value in the server.

The combination of a

- one-offset read error
- a slave/server that does not object to reading 16 bits from one float and 16 bits from another float
- the wrong word/byte order

will produce a "close, but not the exact value that it should be" because the interpreted result is derived from two different data values, using 16 bits from each.  I've seen this twice before and was amazed that the data value could be so close to what it should be.  

The approach is to read two registers starting with the register lower than the current start register.  (and separate attempt read from a register one higher than the current start register).  Trying word/byte format swapping might be needed to hit the combination needed to give the exact value..

Modscan tells you the data's there, you just need the correct combination of start address and word/byte format.

Thanks for you response, Smiffy.

I will collect the data in hex format and share it soon.

Edit : 1 >> I have done some investigation and now I understand the what is the issue. I have attached the zip file which has old screen shots as well as latest.

Earlier, I was facing formatting issue, so I offset the data set because this started showing values in the DB in correct manner but I missed the fact that 1 byte is OFFSET and all data showing using two bytes from different register.

Now I removed the OFFSET and shown data is not making sense and after some more investigation, i found that this is the issue due to byte order, so I rotate the complete byte order and value got corrected.

This is now a big issue for me because there are 146 energy meter of same kind and I am lost and don't know how to correct this.

Please help and the easiest way possible, because correct every single register is not possible to do. 

smiffy

I was going to say rounding errors, but the errors look a bit too large for that.

50.2150 has a hex representation of 4248DC28 (or 4248DC29 rounded)
The value seen on the 1200 is 49.82227 which has a hex repesentation of 42474A01
This a nowhere close.

Likewise

69764.7013 has a hex representation of 47884259 (or 4788425A rounded)
The value seen on the 1200 is 69812.4 which has a hex repesentation of 47885A33
This a nowhere close either.

 It would be interresting to see the raw Modbus hex register values to see what was actually sent over modbus.