Grant chain (was: New 11/24, what to do first? More info.)

Strictly 'NPR wires' shold be 'NPG' wires. NPG stands for Non Processor Grant (and NPR is Non Processor Request). It's much the same as DMA. On the Unibus, an interrupt goes like that : 1) The intterupting device assets one of the Bus Request (BR) lines 2) If the priority levels is set appropriately, the processor (or more strictly the arbitor) asserts the approriate Bus Grant (BG) line. Now the BG lines are not bussed between all the slots. The arbitor drives the inputs on the first slot, the device in the first slot drives the inputs on the second slot, and so on. If a device is not the source of the Bus Request, it passes the grant on, If it is, then it handles the interrupt, and doesn't pass on the grant. This is a sort of priority scheme, devices neare the arbitor have higher priority. 3) The interrupting devices asserts SACK (Select ACKnowledge). The arbitor de-asserts BG. The interrupting device now has control of the bus 4) The interrupting device now outputs a vevtor onto the data lines, asserts the Interrupt line, and asserts MSYN (IIRC). The processor asserts SSYN to say it's taken the vector. 5) The device deasserts SACK, the processor now has control of the bus again, it executes the interrupt service routine. Non Processor Requests are similar. A device asserts NPR, the arbitor asserts NPG, the device asserts SACK, at which point it has the bus and can output addresses, data, etc to access memory. Now, the problem comes in the fact that the Grant lines (there are a total of 5, 4 BG's and NPG) are not bussed, they're daisy-chained. If you have an empty slot, you need to link the input contacts to the output contacts on that slot, so as to pass on the grant lines to the next slot on the backplane. Originally, there was no way to do NPR operations from a single-slot Small Periperhal Controller. So there were only the BG lines, they are on part of connector D. DEC made a little sqare PCB with some shorted-together fingers on one side that you put into connector D to link the BG Inputs to the BG Outputs. That was the original Grant Continuity Card. Soon afterwards, DEC put the NPG line on the Small Periperal Controller SLot, using 2 contacts of connector C. Since the grant continuity card didn't fit into that connector, and since NPR devices on one card were not common anyway, the NPG signal was passed on by a wire-wrapped jumper on the back of the backplane. It was fitted at the factory, you cut it (carefully!) if you put an NPR device in that slot, you re-wrapped it if you took the NPR decice out). Much later on, DEC made a dual-height Grant Continuity card which fitted into both connectors C and D. Needless to say this one links over all the BG and the NPG signals. But the problems are not over. Many older Small Peripheral Controller cards that do not do NPR trnasfers have nothing connected to the NPG In and Out fingers of connector C. If you put one of those in a slot, you have to put the wire-wrapped jumper in place, or solder a wire on the board between the appropriate fingers. -tony