Communication with modbus devices

Hello All,
My name Peter, I am currently exploring what LabVIEW has for instrument communication. I have explored a little on NI-Visa and used it for instrument communication through USB and Ethernet. My next task just now is to see how I can communicate with Modbus devices. I have done some background reading on modbus communication protocol and now have some level of understanding of what it is about. I hope to understand more as I continue to read more materials.
I came across an NI-Tutorial titled Connect LabVIEW to any PLC with Modbus. It is about communicating with a networked PLC using modbus. I followed all the steps described for creating Modbus master I/O Server, Binding shared variables to Modbus Address through the I/O Server and writing to Modbus Addresses in LabVIEW. On running my VI to deploy the shared variable, the following error occured.
Can anyone please guide me on what to do because I do not know exactly what is going ON. I engaged in this excercise with the hope of getting to understand more about Modbus communication and then see how I can apply thesame idea to communicate with the Modbus device available in my Laboratory. Please note that I do not yet know much about modbus communication and I am just learning about it now. I do not have any PLC connected to the network. Could that be the reason for the error? If anyone has got any other relevant document to help me get started with modbus communication I will be happy to have them posted here.
Thanks very much for taking time to read through my post message. Hope to hear a quick response from you.
Regards
Peter

Hi SmithD,
I would like to say thank you very much for making out time to respond to my queries. I have done as directed but still not getting result. Following the wire mode port setting configuration, I stopped the process and then selected the RS485/wire4 as the wire mode. On running the VI, an error pops up with the information that the Visa resource is Valid but the port cannot be accessed. I noticed that If I undeploy the shared bound variable the error does not pop up anymore. I concluded that maybe I was not supposed to have added the VISA configure Serial port in the first place.
Kindly tell me what to do.
My task is to read the holding register from a Modbus device. I have the register map with me, and know the TCP and RTU settings for this device. After the attempts I have made so far I was forced to believe that perhaps the device was not responding. I then resolved to using the QModbusMaster which was previously used to read the holding register. For reasons I don’t not know, It worked well with the TCP communication and returned an error that read " slave threw exception > unknown error". With that, I am now sure that the device is functioning properly, at least with the TCP.
Having obtained result from the TCP using QModbusMaster application, I want to simply do the same using LabVIEW.
Some few questions for which I would want to ask for clarification are:
1.) When using the Modbus Library, do I need to create both master and slave instance to be able to read the holding register? My attempt was to create a Master instance with the RTU parameters. The starting address and number of registers were specified at the input terminals of the Read Holding Register VI. This VI immediately followed the Create Master Instance VI. I was expecting that that would read the information contained in the specified registers and output it at the register value terminal. One challenge with this attempt was that with RTU it didn't work. I tried selecting TCP as the VISA resource. But the device was not showing on the list. I went to MAX to create a new VISA TCP/IP resource under the network devices but MAX could not detect the presence of the device. I am now thinking maybe the device is not supported by NI-VISA and so it will not be possible to use the Modbus Library with it.
2.) If the device is actually not compatible with NI-VISA, can DSC I/O server be used to read the information on the holding register?
3.) Do I need an intermediate device between a Modbus device and the PC to be able to read its holding register?
Please if anybody has got a good suggestion as to what to do to get my task achieved, kindly leave me a post.
Thanks
PETER

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