AB Micro850 modbus and double words/floats

This is all pertaining to the Micro850 and modbus. A couple things:

1.) How do you read double words? I can read/write words but there's no good way to read double words. I found a UDFB called "words into real" but I can't just read those 2 words, put them together, do a "any to dword" conversion and expect a dword. It seems the formatting is different.

2.) How do you write the dword and float? It seems I have to somehow break apart the dword/float into 2 words, send those out as words, then have the slave put the words back together. Is that right? But how?

I've been using this tech note to help me:
https://rockwellautomation.custhelp.com/app/answers/detail/a_id/1032507/page/1

Please advise. Thanks.

What are you using for the Modbus Master (Client)?
Sometimes there is code already written to do the functions that you are asking.
Example: AdvancedHMI
https://sourceforge.net/projects/advancedhmi/
Here is how to read a 32 bit number or float using AdvancedHMI and Modbus.
http://www.plctalk.net/qanda/showthread.php?t=100806

Regards,

gclshortt said:

What are you using for the Modbus Master (Client)?
Sometimes there is code already written to do the functions that you are asking.
Example: AdvancedHMI
https://sourceforge.net/projects/advancedhmi/
Here is how to read a 32 bit number or float using AdvancedHMI and Modbus.
http://www.plctalk.net/qanda/showthread.php?t=100806

Regards,

Click to expand...

The master is a Micro850 from AB.

You probably have a "endianness" problem, modbus doesn't define in which order double word values are send, some plc put the high 16 bit word first, others do the reverse.
In some place your modbus driver should have the option to reverse word order.
The only data type supported by Modbus (both RTU & TCP) are 16-bit words.

The way how these words are interpreted at both ends of the connection does not depend on Modbus.

Modbus does not define how a floating point number is transmitted,
neither a 32-bit integer, neither a character string encapsulated in the 16-bit words.
Hope it will help

You can use the COP instruction. There is a good guide in the 850 manual.
http://literature.rockwellautomation.com/idc/groups/literature/documents/qs/2080-qs002_-en-e.pdf

Basically you split the DWORD to 2 WORDs send and the joint them together again at the other end.

Heres one I did with 5000, so looks different, but the principle is the same. I am reading a HWord and a LWord from a Lenze drive and making a DINT.