Whats the purpose of the sync rtu client? about tokio-modbus

[bootrecords]

After looking at it a bit more in-depth, I have to concur that the design is somewhat awkward. Both the TCP and RTU sync clients internally wrap around a tokio runtime that is always blocked on any call to the sync context, but under the hood everything in this crate uses the async Modbus engine.

From what I can tell your example should run just fine... all calls that require a runtime should be piped through one, though one thing I found peculiar is a call to Serial::from_path, which eventually resolves to a default tokio-net::util::PollEvented that has not been registered with a reactor. However, it may require one in order to handle the later read_input_registers call here.

We have a very similar sync RTU example in #72 (and #76), but I haven't run it against any device so far. The above issue may be alleviated in the newer version of tokio-serial that is being used alongside the tokio v1 upgrades - someone may want to check that sync example against an actual machine, though.