First VAX-11/730 VMS Boot! (was: Re: VAX-11/730 %BOOT-F-Unexpected Machine Check)

(Please try to avoid paragraph-length lines.) I once had a machine in a case it wasn't designed for, leading me to want to extend a DA-15 AUI connector. I experimented with plain untwisted, unshielded, un-ground-planed 15-pin ribbon cable between the machine and a UTP transceiver. I found that a foot or two of ribbon cable made no visible difference. As an experiment, I then tried about thirty feet of ribbon cable and got ludicrously high error rates. On calculating, I find that, at 10Mb, bits are about 65 feet long (at that sort of frequency, signals in copper travel by skin effect at roughly 2/3 c), and, since it's Manchester encoded, the underlying data rate is twice that, meaning a transition potentially every 30ish feet. A cable two to three feet long is short enough for everything to settle in plenty of time even if the impedances are mismatched, but one thirty feet long is about the size of the underlying bits and it's entirely understandable that it would cause trouble. So, if you can hang a transceiver off the back of the machine with little-to-no drop cable length, you can probably get away with a few feet of pretty much anything, but if you want to run any significant length of drop cable, you may well need to treat it as a transmission line and worry about characterstic impedance and shielding and such. But it's worth experimenting with anyway; it's possible the cable you have has the right impedance or some such! /~\ The ASCII Mouse \ / Ribbon Campaign X Against HTML mouse at rodents-montreal.org / \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B