Noel Chiappa wrote: > > > From: Peter Coghlan > > > Can anyone suggest an existing, simple QBUS device that I could study > > the documentation of to figure out what a basic QBUS device needs to > > have and to give me some ideas on how to implement one? > > Depends. Do you want to be able to do interrupts? Do you want to be able to > do DMA? Each is a significant increment in complexity. >
Interrupts would be great to have and DMA might also be useful down the line but I have a much better chance of getting somewhere with this if I keep things as simple as possible for now anyway. > > Later DEC QBUS devices may not be the best things to look at, since they tend > to use special DEC QBUS control chips (I'm _not_ talking about bus > transceiver chips here) which are of course no longer available. > A single chip plus tranceivers solution would be ideal but I couldn't see it being that easy :-) > > If all you want is master/slave (i.e. the ability to read/write registers), > try this: > > http://ana-3.lcs.mit.edu/~jnc/tech/QSIC/test.pdf > > It implements a single 16-bit register. (Changing it to support a single > block of registers would of course be trivial.) > This is pretty much exactly what I was looking for. Thanks! > > The switches (and associated comparators) in the lower left allow one to set > the bus address it responds to; the 3 latches on the right hold the register > contents; the drivers/buffers below them drive LEDs to display the register > contents. The control logic is about as simple as it can be; one latch, and a > couple of gates. > I've studied the circuit a bit and it seems pretty clear what most of it is doing. However, I am puzzled by the BPOK and BINIT signals being connected to U7 even though they do not seem to get used for either input or output. I wonder is this just because two tranceivers were left over and they might as well have something connected to them that might come in handy later or is it because I am failing to understand something properly? > > You should probably read the QBUS description in any QBUS PDP-11 manual > before attempting to understand it, but having done that, it should be pretty > self-explanatory - the signal names should clue you in to what they mean. > Thanks. I'll do that. Another thing I noticed is that there are rather more than 50 QBUS signals listed on the top right of the circuit diagram. I guess which ones are omitted from the 50 pin D connectors will become apparant when I find a pinout for those connectors. I suppose the power rails and those labeled "spare" are likely candidates for omission. I was hoping that there would be lots of signal pairs like SCSI and some 25 pair telephone cable I have on hand might be suitable for the cabling but it doesn't look like that is going to be appropriate. Regards, Peter Coghlan
