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Just curious, but why bother when there are multiple native LabVIEW implementations already? There was the "original" NI library , the LVOOP NI implementation, and I believe at least one third party implementation.

Basically the RTU slave should continuously check its serial port, and if there is any data there it should recheck the port until no new data has been received for those n milliseconds. Then it should process the received message. If the message is intended for a different ID it should just discard the data (clearing its serial buffer for another message). Using 3.5 character silence time can be a bit challenging yes. Windows at least seems to add delays every now and then (it's not deterministic...), so even in a tight loop you can get into that kind of territory (typically you will see this as a problem that increases the longer the Modbus messages are, as the probability of an incorrectly detected halt will then increase...). Luckily most of the time the Modbus Master can be configured to put more silence between polls (interframe delay or similar configuration), making it easier for slaves to clear their input buffer between polls to receive separate messages. We have dealt with most of the control systems used in the oil and gas industry over the years, and have never had an issue after the initial setup (All of them can extend the silence time if needed, and/or chop the messages into shorter ones). Instead of 3.5 byte times at a baud rate of 19200 for example, we might end up using 10-30 ms.