Problem: I'm trying to communicate with a 2 speed fan motor controller using modbus protocol. The fan has 2 speeds (low/high) The problem is how the motor controller works. The controller uses 2 different slave id's and the registers for the slave id's are the same for both. The registers are as follows Slave Function Register Signal Descrip 3 4 30286 Fan S13 On High 3 16 40003 Fan S13 Stop High 3 16 40004 Fan S13 Start High 4 4 30286 Fan S13 On Low 4 16 40003 Fan S13 Start Low 4 16 40004 Fan S13 Stop Low i'm trying to create 1 vi that can communicate with each one of the registers across both slave ID's. I basically want to be able to create a front panel with 4 buttons (start/stop) and 2 status lights for high and low. I started with the NI example serial slave for my other controllers and i was able to maniplulate it to my needs when using a single Slave ID but Fans require multiple slave ID's I thought about using 2 Vi's running at the same time but I dont know how to communicate between them the correct values. I also had the idea of maybe creating a Subvi but that wont work becasue the Vi's have while loops in that vi would then prevent the original vi from continuing. Background: Windows XP Version - Labview 8.1 Modbus RTU over Serial NI Modbus Any help would be appreciated.

Hey Guys, I appreciate all the help. I found a solution and i wanted to post wanted to post my solution here just in case someone in the future runs into the same problem i did, they will be able to find an answer. Here is a quick rundown of the problem again Problem: I was tasked with emulating an entire engineering plant (pumps, valves, fans, engines, etc) I had to pretend to be all the pieces of equipment so the PLC's which will be actually controlling the equipment can be tested and debugged prior to being sent out to the real world and installed. All the equipment i had to emulate, communicates over serial (RS485) using the Modbus Protocol and some the equipment had multiple slaves in them, that needed to communicate over one RS485 cable. by default, you can only communicate with 1 serial port and 1 slave ID at a time. The Solution: The first thing i did was get a more modern embedded controller for my PXI rack. this came with LabVIEW 2014, which was new enough to allow me try 3rd party Modbus libraries. I was not able to get any of these to work out of the box. I also tried the latest NI Modbus library but had the same issue. The solution came when one of the third party companies I reached out too, suggested I try a virtual serial port emulator. After trying a few emulators i discovered Virtual Serial Port Driver Pro by Eltima. I used that software to merge the real port with 2 virtual ports. This way each Slave ID would connect to a different virtual port and both data would go out over the real port. LabVIEW could not connect to the real port because the Eltima software was using and that’s why I had to create a pair of virtual ports. Other possible solutions i was looking into but stopped perusing was a serial to Ethernet idea where i would create a virtual Modbus TCP server on the console and then all the Modbus programs would connect to it over TCP and then all the data would be forwarded to the serial port and vice versa. I had some leads and idea's but i was never able to pursue it.

LabVIEW

Modbus Slave with multiple slaves in one vi.

Solved!

thank you for your advice, it's working now

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still working but this could work. The slave address is not passed into the VISA config serial port. Then each time a new query is sent the first 2 bytes are the slave address. Read that with VISA read and use that to enable a case in a case structure.

I am aware its an old thread, but I guess I am not the only one still reading it