Labview modbus rtu RS485 communication, logging scale net weight

[EricDin]

Hello,

I have a K-tron K-sampler scale that reports scale Net Weight every 1-second continuously.  We are trying to log the Net Weight in LabView.  With the supplied scale manufacturer (k-tron) program, the scale cable is RS485>USB to communicate to a laptop for Net Weight logging via Modbus RTU protocol - (19200, Even, Node address = 31, Register 40050/51 IEEE Float NetWeight).  Has anyone had experience using the Labview (we have Labview 2017) Modbus API (not DSC module) to communicate via Modbus rtu in order to log data?  Thanks

[RavensFan]

Sure.  There are a couple Modbus libraries you can use.

I started with http://www.ni.com/example/29756/en/ and still use it.  There is a newer version out as well you can get through JKI VI Package Manager called "NI Modbus Library".

For the older library, I don't use the Init Port it comes with.  I found it didn't set the stop bits correctly for some of the devices I talk to.  I just use the the regular VISA Serial Configure to set the port.

With registers 40050/51, you'll want to use modbus command 3, Read Holding Registers.  A quantity of 2, with a starting address of 49.  (Drop the 4 from the front of the manuals register address, and subtract 1.)  Now you'll have an array with two elements.  You will need to typecast that to a single precision float.  You may need to swap the elements depending if the most significant word or least significant word comes first.

[EricDin]

Hi RavensFan,

Thank you very much for the quick reply.  I am just wondering if you may know which Modbus library/VI would work best for our scenario.  The Modbus communication will send a 'Net Weight' float value every 1-second to LabView. 

We are also sending 3 types of data -  timestamp, real/float (~80 data points), and binary data (~20 data points) from an OPC server to LabView every 1-second as well.  Originally we attempted to use the 'Write to Measurement' express VI but after 5 minutes of logging ~100 data points per second the data-logging slowed from 1-second data points to 2-second logging which is unacceptable.  We are going to try 'write to tab delimited' VI first to see if it can log all 100 data-points on a 1-second basis for our needed ~2 hours runtime.  If this cannot achieve the 1-second logging then we will try the 'producer-consumer' queue VI.

Thanks,

[RavensFan]

I would use the purple library for a new project.

You'll want function code 3 for Read Holding Registers.  Quantity of 2.  Start register of 49.  (For 40050/40051, you want to drop the 4 and subtract 1)