I found this one: https://minnie.tuhs.org//cgi-bin/utree.pl?file=AUSAM/sys/dmr/dz.c which seems to be the one you're rhinking of, or close to it. It actually does use interrupts, on both sides - sort of. On the input side, it uses the 'silo alarm', which interrupts when the input buffer has 16 characters in it. This has the same issue as the silo on the DH11 - if there are less characters than that waiting, the host never gets an interrupt. Which may be why it does the timer-based input check also? The output side is entirely interrupt driven; it _does_ reduce the number of interrupts by checking _every_ output line (on every DZ11 in the machine) to see if that line's ready for a character when it gets any output interrupt, which will definitely seriously reduce the number of output interrupts - but even then, if _one_ line is going flat out, that's still 1000 interrupts per second. Noel