It is not a problem at the moment because I have only two slave devices but it will maybe become too slow if I add others slaves. It works well but it makes the whole process slower (maybe 2 seconds between each acquisition). So I am communicating with them sequentially. I'm not communicating with both devices simultaneously but I put the program in a loop which allows the communication with one of the device at a time and which change the address at each iteration. Start with relatively long delays and then shorten them up until problems start appearing again. Try putting delays between messages to see if that helps. You may also need a slight delay between each set of transmissions because some devices are "deaf" for a short period of time after a message was sent (even it if wasn't for them). It is up to the "master" (your Labview program) to query one device, get its reply, query the second, get its reply, and then repeat. On RS-485 however, you're working directly on the bare hardware. If you were using Ethernet TCP/IP it would take care of those coordination issues for you. If two devices try to communicate (reply) at the same time they will interfere with each other. There is however no mechanism built into RS-485 to allow multiple devices to communicate at the same time without stepping on each other's toes. You can have up to 32 nodes on standard RS-485, and that limitation is due to the electrical characteristics of the chips and cabling. This is in fact usually the reason why RS-485 is used at all (instead of RS-232).
Modbus/RTU on RS-485 can definitely be used with multiple devices.
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