I';ll try a one-word explamation : 'Grants'. This is rapidily becomign FAQ for this list... Whenever a PDP11 misbehaves with an M9302 terminstor is fitted, but works without a terminator (or with an M930). then the most lielly problems is an open-circuit grant chain. The grant signals are active high (strangely, since jsut about everythign else on the Unibus is active-low) and are not bussed, they are passed What should happen is that the arbiter (part of the processor board) asserts the appropriate grant signal in response to a bus request (providing the prority level is high enohgh). Each devive passes the grant on to the next device until it gets to the tdevice that asserted the reqeust i nthe first place (if there's more than one such device then the one nearest the processor does this). This device now esentially has control of the Unibus. As a result, the grant signal should never get the full length of the unibus, it should never arrive at the terminator. But in soem caes (if a device deasserts the request line), it can do. The M9302 has a little logic on it to assert the SACK signal under such contitions, basically telling the arbiter to remvoe the grant. The M9302 then deasserts SACK, the machine carries on with the CPU in control of the bus. The problem comes if there's an open-cuircuit anywhere in any of the grant chains (remember these siganls are passed on by each device, so if yoyu have an empty slot you have to fit a 'grant continuity card' which shorts the grant in to grant out pins). In this case, because the signals are active high, the terminator sees an asserted grant line (things flaot high,or are pulled high by the terminstor) and the M9302 will assert SACK. The CPU can do noting to deassert the grant, it's not sseritng it in the first place, and ven if it ws, the open circuit would mean the terminator would stil lthink it was asserted. So SACK remains asserted, the suystem hangs. There are 2 types of grant continuit card. The later oens are dual-height and go in conenctors C and D. These have a normal module handle and are easy to fit. The older ones are little sqaure PCBs that go in conencotr D only with the traces towards the /unibus out' end of the backplane. These conenct the 4 bus grarant signals only./ The nonprocessor grant (DMA grant) is on slot C, pins CA1 and CB1. and is completed by a wire-wrapped jumper on the back of the backplane. Of course a faulty device in the system could be forcing a grant to be assertted when it shouldn't be, but this is less likely So : Check there's a grant contiuity card in every empty SPC/MUD slot. Check the NPG jumpers are fitted if necessary. For each device card, check there';s a bus priority jumper plug fitted and that it's making contact (the grants pass throug this too). Check there's the NPG jumper on the backplane if the device doesn't do DMA. If you can't fidn the fault,. start at the terminator. Find out (logic probe) which grant is assserted. And work abck form there. Where is it not being passed on correctly? -tony