Ah, thank you Paul, for sharing this story. Postings like this make cctalk worth every penny, IMO! Regards, Freek. -----Original Message----- Date: Tue, 08 Jul 2025 21:30:36 -0000 From: paul.kimpel(a)digm.com Subject: [cctalk] Burroughs backplane story, was Re: Another 780 backplane story To: cctalk(a)classiccmp.org Message-ID: &lt;175201023609.1228.2550589532797629102(a)classiccmp.org&gt; Content-Type: text/plain; charset="utf-8" In early 1971, I started working as an on-site software tech on a rather large (for the time), dual-processor Burroughs B6500. It was early days for that system, and we had a lot of problems with it in the field. We got those ironed out pretty well and eventually got the system up to an acceptable level of operation. <<< snip>>> We finally deduced that the problem had been present since the system left the factory floor. The original I/O software had been so poor that the system was seldom (if ever) able to initiate more than one I/O to disk at a time, but the new version we had recently installed was really good at initiating multiple I/Os. Mux 1 had a lower selection priority than Mux 0, so under the old software it was seldom selected, and perhaps never so for tag transfer I/Os, which are relatively rare. The new software allowed the system to get busy enough that Mux 1 started to be used a lot more often, and eventually it got busy enough that a paging I/O for that master PCW array got scheduled to Mux 1, and the system just didn't survive for very long after that.