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Once, long ago, I worked at a company that made hardware that plugged into a Unibus backplane. This backplane was used by DEC (an early minicomputer) systems. The backplane, much like those of today, was a series of card-edge connectors. We had a product which communicated across this backplane using programmed I/O with a driver installed into the kernel of the machine (running RSTS-E). We'd developed a new version which was able to use DMA to directly access the buffer memory of the system processor; it was pretty small, only 256K bytes. We had a system we tried to install this on in England, and there was some problem we could not figure out. It just wouldn't work. Now, the installation procedure involved the backplane intimately. In those days, the backplanes weren't a printed circuit card, but rather a series of connectors connected by wire-wrap wire. To install a DMA card, you had to unwrap the DMA grant line from the backplane. We even had special cards to insert which jumpered it back if you needed to remove one of the DMA cards. After several days of trying various things, we gave up. About 11 years later, the installer and I were talking over beers, and we finally figured out the problem. Of course, by that time we were both working at a new company. The issue was that the grant line was damaged or removed in another slot of the backplane, but wasn't correctly jumpered back. When they moved around a card to 'test' the slot, it didn't use DMA, and so worked fine. |