Probably true. I mentioned it because I've seen talk of "four-
wire RS-485", which is probably just RS-422, but I haven't
examined it closely enough to know that it isn't some hybrid beast.
There are. Greyhill uses them on their microDAQ controllers. For
hardware it is 422 but since it is a daisy chained buss it has a
485 command structure to take care of who is talking. They also
can be simply switched to a 2 wire interface for true 485
behavior. Basically it is a hybrid beast and just means that your
485 software can run with a 422 port.
That sounds like regular multidrop 422; very common in older
industrial control systems.
Going back to the OP's question; RS-485 implies a protocol of some
sort, and quite typically requires a mechanism whereby the protocol
stack can communicate whether a byte is address information or data.
The former can, to a degree, be handled with a line discipline (but I
don't recall whether Darwin supports loadable disciplines), but the
latter does not work well with the current serial family architecture
due to it being byte-oriented. You would end up tunneling address
tags, either via escapes or using the mechanism I applied long ago to
the FreeBSD RS-422/485 on 16550 driver where the stream is actually
word-sized data with bit 8 being the address flag.
