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SCSI

Jon Guidry shared a link.
1 April 2019 ∑
Excellent read. Any TOPS10 Systems Concepts folks that might be able help this guy decode the disk images he has for the SC40s?
How I Couldnít Stop Poking at Mysterious CompuServe Server Hard Disk Images MEDIUM.COM
https://medium.com/@mpnet/trying-to-make-sense-of-compuserve-server-hard-disk-images-posted-on-archive-org-b1c62ce6012b
How I Couldnít Stop Poking at Mysterious CompuServe Server Hard Disk Images Preface
11 9 comments
Richard Sexton "And if the filenames are to be believed, they were taken from SCSI drives not existing until 15 years later. How could this be?"
Because SCSI was around for a while before it became satandardies as scsi. Plus before that it was called SASI = "Shugart Asssociates Systems Interface". The "Shugart" part bothers other drive companies so the name was changed to SCSI = "Small Computer Systems Interface".
" I was quite surprised by all the advanced features. Some of them were very much ahead of its time." The other way of looking at this is Widows blows dead goats. 2
Charles Lasner I was around when the rigged "SCSI" spec came out. It had the not-o-hidden agenda to make the real Shugart stuff obsolete, but fortunately it was defied by large enough vendors so that if you knew what you were doing you could make what you could CALL "SCSI" and yet actually be SASI and not lose compatibility. There were some interconnectivity issues depending on vendor's equipment both in terms of host adaptors as well as controllers.
In a medium-sized nutshell here are the issues:
Charles Lasner 1) SASI does not have the concept of the ATN ]attention] signal. Most simply do not need it.
2) SCSI requires the "battle of the host adaptors] to set their priority on the eight logical unit lines [if you have 16, the first 8) and the one closest to ID 0 "wins". Controllers have to honor the winner and butt out if not. 3) Because there has to be a host adaptor ID, that means no more than 7 controllers while SASI can have 8.
4) Savvy manufacturers of peripherals are not blindly looking for the ATN signal, it is OPTIONAL just not so on the paper spec.
3) Because there has to be a host adaptor ID, that means no more than 7 controllers while SASI can have 8..here is only one host, or in other words SASI. Foolish makers swallowed the noise in the paper-spec and become needlessly incompatible with SASI which was the hidden agenda.
One of the early non-adopters was Apple. Gave lip service to the name SCSI and in fact all SASI. I worked for a small company that pointed all if this out to deaf ears, then Apple came in and it flipped 180 which indirectly helped us, so we did SASI and also call it SCSI; if Apple can do that, so can we.
3
Charles Lasner A neat trick of the paper-spec is that a host can pretend to be a controller and inter-system transfers could happen. In practice, few systems were ever connected to each other.
This all became moot when the big makers came along like Adaptec and PCs. Apple stopped making bit-banging SASI called SCSI and their sysems, being really Intel and PCI bus just used the Adaptec cards without a BIOS ROM [only needed for MS-DOS].
Charles Lasner On the PDP-8/A, CESI made the MDC8, the best by far disk interface for a PDP-8. It used an OMTI SASI [called SCSI] controller and MFM disks larger than anything DEC did and 5-1/4 AT-type floppies only with 26 sectors of 256 bytes each. Forget RX anything, these were over a mB diskettes.
my system has one or two ST-4096 disks on it which were 80 MB but because six-bit bytes and formatted 256 bytes/sector. Wiring handlers for it was the easiest job ever. You could transfer up to 32K at a time, but generally no more than 4K at a time for the usual 4K/8K single-user systems. But you could write a damn-fast backup or restore facility using a bunch of floppies like nothing ever seen before. Needed to be on an 8/A because it was implemented on one hex-wide board that connected 50-pin SASI/SCSI cable to the external controller in a box where the MFM disks were.
Charles Lasner This was concurrent with DEC's meager offerings like the RK and RL disks that were 2.5 = 10 mb each and very large. We also supported SyQuest removable 40 MB minicartriges That was when DEC could not really compete with its own third-party vendors. You didn't need lock-step compatibility, just handlers for all systems . I wrote all of them including the diagnostics/formatter program. 1
Jason Caldwell Worst thing about SCSI was knowing or remembering to terminate the chain. If you failed to put on a SCSI Terminator your data would become corrupted. 1
Jon Guidry Hey, if you guys can head over to that Medium article if you can offer more info and comment on it, Iím sure the guy would appreciate it. Prior to me commenting on the article, nobody else had 😞 Thanks, all!
Oliver Klink Reading this felt a bit like coitus interruptus...
https://www.facebook.com/groups/1233645553343772/permalink/2632619883446325/
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