LabVIEW
connecting HMI controller using Modbus
Solved!
Hi Labmaster,
read the manual more carefully: these are boolean values and the notation says "Register 40003, Bit #14"…
First, there is no need to implement Modbus yourself and I would suggest not using the old NI library, because it's not very good. There are better implementations of Modbus around, like this one - https://sine.ni.com/nips/cds/view/p/lang/en/nid/214230
There are probably others too, but I don't have much experience with any of them.
For the function code, you want to read or write holding or input registers registers, because the addresses in your device are between 30000 and 50000. See for example the table here - https://www.simplymodbus.ca/FAQ.htm#Stored
Note that usually the actual address you will put into the specific function starts with 1, so if you want address 40013, then it will usually be read holding registers with address 13.
The number of registers you want to read depends on the range you want. If you want to get data from register 1 and 8 in the same call, then you will need to set 1 as the start register and at least 8 as the number of registers.
The 40003.14 is not actually a float, because its data type as marked as B, which means boolean. What they probably mean is that the register is actually a bitfield and this is bit 14. You can convert these using Number to Boolean Array.
The ones that are floats are actually a good question. Each Modbus register is 16 bit and the commonly used floating point format use either 32 or 64 bits. In this case, it's likely they're using single precision floating point, which is 32 bits, and you will need to read two consecutive registers and then convert them to a SGL value using the Type Cast function. For example:
Depending on exactly how the device formats these values, you might need to reorder the bytes before converting. This can be done, for instance, by using Split Number and Join Numbers.
