Hi, sorry about the delayed reply; been dealing with this: http://ana-3.lcs.mit.edu/~jnc/jpg/backoak/WholeTreeS.jpg The cranes arrive tomorrow... and Hmm. Well, that shoots down the simplest theory; that a CPU BDAL (or perhaps BDIN) driver (technically, a transceiver) chip is bad; if you're seeing any activity at all on a line, the driver must be working. So, either both console cards have an issue, or something more complex is going on. Here are some pictures to show you what you should be seeing, and what I'm seeing with an LSI-11 with no console card. First, normal operation: http://ana-3.lcs.mit.edu/~jnc/tech/pdp11/jpg/BSYN-BDAL_OK.jpg The top trace is BSYNC, the bottom BDAL10 (which should be asserted for 0177560, the console CSR's address; it's the 02000 bit). The timebase on this one is 1 usec per division. As you can see, it's in a tight loop reading that register. The QBUS spec shows that for a DATI cycle, the DAL lines are set up before BSYNC is asserted (falling edge here, since the bus lines are inverted). BDA10 is indeed asserted (low) when that happens; shortly after it goes back to 0 (high) so that device can put its data out on those lines. It stays high, so that bit in the CSR must be 0. OK, now a picture of the bus with no console card: http://ana-3.lcs.mit.edu/~jnc/tech/pdp11/jpg/BSYN-BDAL_NoCon.jpg It's a bit hard to interpret what's going on here (note that the timebase is much larger - 5 usec). The long assertion of BSYNC is undoubtly the CPU trying to get the console CSR to respond, and eventually timing out. Not sure what the short assertion following it is - without looking at the ucode for the ODT, there's no way to know what the CPU's doing. Even harder to understand is what the BDAL line is doing. It looks like it's un-asserted (0, i.e. +3V) on the falling (electrically - rising, logically) edge of BSYN (which would be incorrect - see above). And then it hops around while BSYNC is asserted, which makes no sense at all to me. At this point, my best guess at the most likely cause of your problem (given the 'all the lines are doing stuff') is that both console cards have issues. Tomorrow, when I'm not outside, I'll try and look at some other BDAL lines and see if they are doing the same thing with no console card in. Noel