Negative values in Modbus Master

Joshua37 · Dec 16, 2014

Hi All,
I am using Modbus Master program to get data from DEIF controllers for 4 DG's to a CompactLogix L32E on the serial port.

While everything else is working perfect I am getting incorrect negative values of KWH except for one DG.

Function code:4 (Input Register 537)
Actual Data VS Modbus Data
DG1 =51287 / -13921
DG2 =9789 / 9789 (Correct data)
DG3 =46783 / -23993
DG4 =41324 / -24957

While using Modscan I am getting the correct values via Modbus.

Could somebody help ??

Thanks

rootboy · Dec 16, 2014

It looks like your DG uses a 16 bit unsigned word, while on the Logix side you are using a signed 16 bit word. What are your data types?

Joshua37 · Dec 16, 2014

Int[600]

rootboy said:
It looks like your DG uses a 16 bit unsigned word, while on the Logix side you are using a signed 16 bit word. What are your data types?

Its the default data type from the Modbus Master program from 5000 sample programs.

Mod_Data_InpReg3[xyz]
The data type is INT[600]

Do I need to change the data type?

rguimond · Dec 16, 2014

Joshua37 said:
Hi All,
I am using Modbus Master program to get data from DEIF controllers for 4 DG's to a CompactLogix L32E on the serial port.

While everything else is working perfect I am getting incorrect negative values of KWH except for one DG.

Function code:4 (Input Register 537)
Actual Data VS Modbus Data
DG1 =51287 / -13921
DG2 =9789 / 9789 (Correct data)
DG3 =46783 / -23993
DG4 =41324 / -24957

While using Modscan I am getting the correct values via Modbus.

Could somebody help ??

Thanks

You'll get negative values returned if they're over 32767 because you are probably using signed integer type. MSB typically equals sign. If it's 1, number will be displayed as negative

bernie_carlton · Dec 16, 2014

You'll get negative values returned if they're over 32767 because you are probably using signed integer type. MSB typically equals sign. If it's 1, number will be displayed as negative

That's what I thought except that the negative values shown don't represent the large positive number interpreted as signed.

Please post a link to the sample program or its documentation.

Can you post a link to the DEIG manual which describes the Modbus registers? Exactly which Modbus registers are these reading?

rguimond · Dec 16, 2014

bernie_carlton said:
That's what I thought except that the negative values shown don't represent the large positive number interpreted as signed.

You're right! Even if you invert the bits for the most significant register, the numbers just don't compute.

I'm stumped, but I would sure like to see the solution to this one.

rguimond · Dec 16, 2014

These are long shots, but are you sure you're not populating the values of the registers in the L32E within the program? Maybe you're writing values from another device? Maybe you're MSGs are overlapping?

rootboy · Dec 16, 2014

Yeah, I saw that too (that the unsigned values don't match up to the converted signed values). But it sure looks like the problem is originating with the sign bit.

So, try loading these values in different data types (Unsigned, Real), to see what happens.

And it should be possible to tap into the Modbus network and do a capture of the traffic and post it here. We can then convert these messages by hand to see what the DG's are sending out.

BobB · Dec 17, 2014

I must say I have had some funny results from DEIF controllers in the past but I heard they had fixed the problem. Hopefully they have - good luck. Always use Woodward these days then no issues.

Negative values in Modbus Master

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