On 10/05/2017 01:50 PM, Chuck Guzis via cctalk wrote:
You could well be right--I do recall that there was "Mac SCSI" and then the slightly different "Everyone else's SCSI". I ran into this when talking with some SMS/OMTI engineers about an ST506-to-SCSI bridge board that I have. Their reaction was "Oh, that's Mac SCSI--you want real SCSI".
Hmm, I was under the impression that at the connector level the Mac flavor still carries all the signals of real SCSI, but leaves out a lot of the grounds so that it'll fit into 25 pins. At the protocol level, it's the same. At the low-level software side of things, Macs could be picky and only talk to a SCSI device which identified itself with Apple branding - which would certainly cause problems in hooking up something like an OMTI board.
I do know that many SASI devices work as SCSI-1 devices. Somewhere, I still have an early PC ISA SASI (not SCSI) adapter for an Ampex Megastore unit.
My understanding there is that true SASI supports just a single target, and so there's no selection phase like there is with SCSI (and SCSI provides an extra signal on the connector uses during selection, which simply isn't there with SASI). However, there seemed to be some significant overlap and blurring of lines between SCSI and SASI, such that some early devices calling themselves SCSI aren't quite - and it's possible that some hardware which talks about SASI actually behaves more like SCSI.
I don't think SASI had any parity support, either - but I think that in a lot of cases early HBA's relied on parity checking in software, which meant that it could simply be ignored.
Lots of early SCSI devices have no support for the Inquiry command, which trips up modern software which expects it - I don't know if it was simply ignored, or if there was a point in time where it wasn't present in the spec, and was only added later.
cheers Jules
