in If you have W4 in, you can do whatever you want on the BDV11, with the front panel LTC ('Aux') switch, etc, and it won't make a difference - the machine will not do LTC interrupts, no way, no how! :-) If W4 is out, _and_ the LTC switch on the front panel is on, _and_ the BDV11 is set up to control the LTC, then the machine will boot with the LTC off, and the LTC can be turned on by the software (by storing a '0100' in 177546). Oh, one other thing to pay attention to: not all DEC backplanes have termination QBUS pull-up resistors. The H9270 and H9373 (normal equippage in the BA-11M and BA-11N boxes, respectively), for instance, do not. Those machines need something at the end of the bus to provide the termination/pull-up - such as a BDV11 or an REV11. And then we get to 18 and 22 bit busses... :-) Noel