MODBUS / ML1400 help?

Nhrafan26 · Mar 13, 2017

I have a device that I am communicating with here that is using Modbus to store values from a gas analyzer and I want to transfer these to a PLC. I have got the 16 bit WORD values to display and transfer successfully.
The one I am having trouble with is I am trying to read a 32 bit FLOAT value for the gas concentration value. I have tried various messages, all with the same result: Bad parameter in MSG reply packet, could not be processed by local processor. Error code hex 45.

Any ideas? Or what more information do you need?

Thanks

Nhrafan26 · Mar 13, 2017

Here's 2 screen shots of the MSG file and the sheet I got from the monitor company...

nhatsen · Mar 13, 2017

Did you try reading 2 INT intead of a long word? After that, you'll need to recreate the 32bit float.

Ken Roach · Mar 13, 2017

I think that making the data table address a Long Integer (4 bytes) and a Length of 2 will make the length request be 8 bytes.

If Holding Register 3658 is the last 4 byte register in the Modbus table, then requesting 8 bytes starting there will probably make the slave device give you an error code.

I would make the target an ordinary N integer element, with the Length of 2, then use COP and SWPB instructions to experiment with byte order to convert it to a REAL.

Or, see if you get lucky with byte order and make the target a Floating Point F element, with a length of 1.

Dravik · Mar 13, 2017

So the number of elements is determined by the destination device. Since you are issuing a read command, you probably want to switch that to 1 if you are using F or L(since those are 32bit by default).

Nhrafan26 · Mar 13, 2017

nhatsen said:
Did you try reading 2 INT intead of a long word? After that, you'll need to recreate the 32bit float.

I set the message up how you said, now this is how the data shows up.
It's under column 5 and 6. How do I get this to a real number display?

Ken Roach · Mar 13, 2017

It helps to look at it as hexadecimal.

24920 decimal = 0x6158 hex
16416 decimal = 0x4020 hex

0x40206158 = 2.5059414 as an IEEE 754 Floating Point value.

So I think the byte order and word order make this simple. Use a COP instruction to Copy your Integer words (starting at the first one) into a Floating Point (F) register, with the Length of 1.

Geospark · Mar 13, 2017

Here is a technote outlining specifically what you are looking for and what the members here are describing...

559679 - Instructions for Modbus device to pass float data to controller in two integers
Access Level: TechConnect

It includes information on an SLC500, a MicroLogix 1400 and a CompactLogix.

Regards,
George

Nhrafan26 · Mar 15, 2017

Ken Roach said:
It helps to look at it as hexadecimal.

24920 decimal = 0x6158 hex
16416 decimal = 0x4020 hex

0x40206158 = 2.5059414 as an IEEE 754 Floating Point value.

So I think the byte order and word order make this simple. Use a COP instruction to Copy your Integer words (starting at the first one) into a Floating Point (F) register, with the Length of 1.

Problem is have you ever tried the COP command from integer to float with a ML1400? It doesn't work. "Operand sizes do not match."

So I used the CPW command instead and got no errors but now I need to figure how to take the number being displayed in the float table and send that number to an integer file without it being broken up.
Right now I have a test message running and am seeing the setting of 20 appear in the float table but when I copy that to integer I see zeros.

Nhrafan26 · Mar 16, 2017

Is there any way for me to take my value and get it to read in this format on a server (00.00) ? I am not sure how taking it from the modbus message, into a N table then to a float table and then into a final N table to transfer to a server will affect my reading.
Anyone?
I have posted the tables in my previous post.

MODBUS / ML1400 help?

Nhrafan26

Member

Nhrafan26

Member

nhatsen

Member

Ken Roach

Lifetime Supporting Member + Moderator

Dravik

Member

Nhrafan26

Member

Ken Roach

Lifetime Supporting Member + Moderator

Geospark

Lifetime Supporting Member

Nhrafan26

Member

Nhrafan26

Member

Similar Topics