CitroJim wrote: 07 Apr 2023, 05:41
I'm delighted you are preserving these Zel
You have the makings of a good museum there
I'm just glad I've had a bit of a kick recently to actually start doing something with them again.
The page for the T1200 is mostly done now. Had a bit of an amusing moment this evening which made me question my sanity somewhat.
I decided looking at one of my photos...
...That it would be helpful for readers to produce an annotated version.
I might do another pass graphically as it's a bit cramped on the left hand side, but that's the quick bit.
This took several hours of looking up part numbers, not being able to find some, reverse engineering a bit, and just generally figuring things out.
Shortly after I'd finished I had a sudden moment of "Wait a second..." Cue me grabbing the service manual. Yep...there's a complete board diagram showing everything in there isn't there! *Bashes head against desk a few times.*
On the plus side, aside from having worded a few things differently, nothing was actually wrong. The way I see it as well I'll have absorbed a lot more of the information than I would have just copying things from the manual. As if absorbing information about the architecture of a system board from 1987 is actually useful...well I guess it is if you're me.
I'd actually quite like to annotated too down photos like that for most of the older systems now I've done one. It's pretty pointless for anything made after the early 90s as so much started to get integrated into single big chips, but on older machines like this where everything is more or less discreet I think it makes a lot of sense.
I also started looking slightly more deeply at the hard disk interface...and discovered that was a bit of a rabbit hole. It's actually kinda fascinating.
This dates back to around 1986...most hard drives were half or full height 5.25" ST-506 drives. 3.5" drives weren't massively common by then, and IDE drives were still very new.
The ST-506 interface isn't exactly well suited to a compact format though...the interface connectors are nearly as wide as a 3.5" drive just by themselves...plus you don't really want to have to dedicate a whole ISA slot to the interface card. So JVC decided to come up with their own solution...the result being this oddball solution which combines both data and power onto a single 26-pin ribbon cable.
The interface is very ST-506-esque, but not quite. The differential pairs used for signalling on the ST-506 standard have been ousted with direct TTL level communication, and several other functions have been added. Including the ability to command the spindle motor to start or stop, a specific line to instruct the head to park, useful sorts of things to have on a drive optimised for mobile use.
The drive itself is a little odd as well...I'm used to ST-506 drives almost universally running at 3600rpm - as at 17 sectors per track, that gave you the 500kbit/second data rate.
This drive runs at 2600rpm...which is a really strange spindle speed. I've never come across one running at that speed before. The documentation showing this seems to be backed up by my actual testing on the machine.
Just quite an unusual setup. Well...it was physically unusual as well, being exactly the same size as a modern 3.5" drive - *including* a protective cage and rubber shock absorbing mounts.
It also has heads which automatically park after a few seconds of inactivity rather than requiring to be manually parked like most drives of the time. I hadn't really thought much about it until today, but this little thing was really quite an impressive bit of kit for its day. Well done JVC.
Might need to do a bit more digging one day and maybe put a bit of a background page up on the technical details of that aspect of the machine. Just find it quite intriguing to see JVC coming up with such a different solution when the rest of the world were going down the road of IDE, or of course SCSI.
I wish you a very quick and full recovery 
