There seem to be a great many models of Unibus and Qbus multi-port async serial boards, which present different register-level interfaces, e.g. for Unibus, DH11, DHU11, DJ11, DM11, DZ11 . Which ones are considered "best", for each bus, for use with a multitasking OS like RSTS/E or RSX-11M+?
Depends on your use case. The mighty DLV11 is nice, but it's interrupt driven so every time you shove a character out it takes bus cycles and every time a character comes in you have to drop what you're doing. The DLV11-J will run down your system if used for interactive users as it's 4 DLV11's.
DZV is a step up, you at least have an incoming FIFO, can program the baud rates in software, and they can afford to buffer a bit. But a bunch of DZV11's can sink your bus as well, so you go to the DHV11.
DHV11 is what you need for a lot of users: Bigger FIFO buffer for input, DMA access to do a lot of characters in a few bus cycles, and oddly enough intelligence to do things like automatically send XON and XOFF without having to bother the CPU. Apparently you can send and receive on all 8 channels at 9600 baud simultaneously which is pretty good.
Going above this you start to get into the DMC11 type boards which have hardware support for DDCMP and the like. That's what you want if you're going to do DECNET Phase III over serial lines. You can do it with the other boards but the CPU is going to have to deal with the framing and whatnot.
But by then you just do ethernet. C
