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Problems reading modbus REAL from energymeters

Anders

Hi!

I´m trying to read values from several Schneider iEM3250 energy meters, all of them giving me the same problem.
Refer to the manual for modbus info: (https://www.se.com/ww/en/product/A9MEM3250/iem3250-energy-meter---ct---modbus/)

For example, on page 60 there's a list of registers for voltages, starting at 3020 and incrementing by 2.
The document says size:2 and type Float32. Each being 16bit value I suppose which then makes sense.

In citect I connect using the MODBUS1 protocol, and addressing them as Holding, function 4 in citect.
So the addresses are 43020, 43022, 43024 ... etc.

All the phase-phase voltages reads good, but only the first two phase-neutral reads good. The last one reports -nan in runtime. (address 43032).

When using a modbus poll program (AVreporter modbus communication tester) I find that the addresses I read from citect varies a lot in value, while the following address stays quite still.
I suppose when addressing REAL citect combines two bytes into a REAL value?

All the addresses that works has a following value of around 17500, but the non-working address has a value of 65470.
I don´t know, but starting to guess that citect expects a signed INT, but gets an unsigned INT a freaks out?

Anyone got an idea?

Regards
Anders

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  • When Citect reads REAL values it uses floating point notation. The two registers are not read as INTs. They are read as a combined single value using floating point notation. The web site https://www.h-schmidt.net/FloatConverter/IEEE754.html provides some help on structure and a converter. You don't mention how the meter is physically connected, whether this is a wye-connected or delta-connected meter. The connection affects the available voltage readings. From your post I am assuming that 17500 is a correct phase-to-phase reading.
    Unless it is specifically required for this meter, I would use MODBUS and not MODBUS1 protocol. The numbered Modbus protocols have specific adjustments for specific needs. Make sure that the adaptation of the Modbus1 protocol is consistent with the meter. The Citect driver help files explain the unique features of the various Modbus numbered protocols.
    How are the other values you are reading? Is it just the phase-to-neutral voltages that are in error?
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  • When Citect reads REAL values it uses floating point notation. The two registers are not read as INTs. They are read as a combined single value using floating point notation. The web site https://www.h-schmidt.net/FloatConverter/IEEE754.html provides some help on structure and a converter. You don't mention how the meter is physically connected, whether this is a wye-connected or delta-connected meter. The connection affects the available voltage readings. From your post I am assuming that 17500 is a correct phase-to-phase reading.
    Unless it is specifically required for this meter, I would use MODBUS and not MODBUS1 protocol. The numbered Modbus protocols have specific adjustments for specific needs. Make sure that the adaptation of the Modbus1 protocol is consistent with the meter. The Citect driver help files explain the unique features of the various Modbus numbered protocols.
    How are the other values you are reading? Is it just the phase-to-neutral voltages that are in error?
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