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Retrochallenge Roundup (and a quick JVC update)

Well, it's the final night of RetroChallenge, so it's time for a closing post. As I'd expected, I didn't get anywhere close completing my goals, with lack of time being the age-old excuse. However, it was still a very productive project and I plan to keep pressing on and making semi-regular posts. Hopefully I can keep the momentum going even through RC is over for another 6 months. Although I didn't get a wiki up and running to store the information I was gathering, it's all up on this blog - so that's a start as far as making it available is concerned.

Other key milestones:

  • I hacked together a working PSU for the V86P - learning a lot about switch-mode PSUs in the process
  • I reverse engineered the output stage of the original PSU, using photos and some measurements being taken remotely, and captured the result as a kicad schematic
  • I'm most of the way through a PCB layout for the charging section of the PSU, which will provide all the necessary voltages to the V86P to run and charge the NiCd pack given only a sufficiently beefy 8.5V PSU. The missing piece is finding a case I like, then I need to tweak the board dimensions to fit
  • II got the V86P running with both its internal HDD and FDD
  • I found datasheets for most of the key ICs in the system
  • I'm about half-way through mapping out the pinout of the hard disk controller to motherboard interface pinout
  • I confirmed the pinout of the 26-pin JVC interface
  • I identified the SHIP_READY and SERVO_GATE lines as signal that the heads were parked and a sector index respectively
  • I identified that DOS sees a 4 head disk with 512 byte sectors, but the disk itself only has one head select line - possibly there's some logic for splitting 1k sectors into two logical heads - developing some basic C routines to call BIOS int13h
  • I've confirmed I can read the ECC blocks from the data block via DOS int 13h calls, which should make reverse engineering the polynomial used by the controller relatively straightforward
  • And lastly, which I'm going to cover below - I think I've finally confirmed the disk is storing 2,7 RLL encoded data
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The JVC 26-pin Interface – Part 2

Welcome to the second part of my adventures with this slightly unusual hard disk interface. The more I dig, the more questions I raise - it's certainly good for raising the overall feeling of confusion. I think overall I'm making progress, but I'm still a long way away from even considering disk emulation (which was the original aim). As a quick recap of my previous posts - I designed a breakout board which brings the 16 lines of the 26-pin interface that aren't power or ground out to a pin header, along with support for supplying external power to the drive. I also got a power supply working so the laptop can run under its own steam, with both hard disk and floppy drive working correctly. I've got datasheets for most of the chips on the controller, and they look to be a standard ST506/412 RLL chipset, although with a few subtle differences and a mystery JVC chip - I've been reading through those and generally learning more about MFM than is probably necessary in 2017, but hey, isn't that what this hobby is all about?

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Retrochallenge Half-Time Round-up

Well, I meant to write this up at the weekend, but that didn't happen. Better late than never - it's time for a half-time round-up, or perhaps a review of quite how much has been added on to my to-do list for this project. It really does seem to be one of those things where it appears complicated at first, then you get into it, and discover there's a significant number of smaller tasks you hadn't thought of to begin with. Despite the mamoth amount of work outstanding, it's actually been a pretty productive first half of the challenge, and I'm certainly enjoying it.

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In which the author comes to grips with the SMPS

Power supplies are probably something we all take for granted. Pretty much everyone knows that if you plug an electronic device into the power outlet on the wall, there's a thing called a power supply that somehow converts the mains voltage to something lower (and safer). I suspect most people reading this post know at least a little more than that, maybe something about transformers. Perhaps like me, you're familiar with unregulated DC supplies (step down the AC voltage with a transformer, rectify it with some diodes, then smooth it with some big capacitors) and linear regulators (take an unregulated DC voltage and essentially dump the excess above the preset output voltage as heat), with a vague awareness that Switch Mode Power Supplies are far more common these days, and much more efficient.

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Reverse Engineering the V86P PSU (Part 1)

My V86P did not come with a PSU - this makes things all sorts of interesting. Whilst 8.5V isn't hard to get from a bench supply, it's not a particularly common voltage for off the shelf PSUs, especially not at 4A (which is what the original unit is rated for). To top things off, I've suspected for a while that a good chunk of the NiCd charging circuitry is inside the PSU, rather than than the laptop itself, but precisely how much I didn't know. I started with a few simple tests with an ohmmeter between the various power input pins, and was able to confirm that two pins on the 6-pin PSU connector are connected to Gnd, two pins are connected together (I know from the site that these are the main 8.5V in), the pin immediately above these connects directly through to the positive pin of the NiCd battery pack, and the last pin is an unknown. The PSU connector is located on a separate PCB under the motherboard, inside a metal housing that makes up half of the battery compartment. A short cable protrudes from this housing which connects to a 6-pin header on the motherboard, and a pin just above the power switch. There's also a separate wire that goes to a ring loop which goes through one of the motherboard screws - presumably chassis ground.

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The JVC 26-pin Hard Disk Interface (Part 1)

As I've mentioned in a previous post, my V86P is the higher-spec model containing a 20MB hard disk. Back in the mid to late 1980s, IDE was still relatively new and most systems were still using ST-506 interface drives, albeit using RLL rather than MFM to increase data rate and capacity. For example, I remember the Viglen Vig II 16MHz 286 we bought as our family PC back in '88(or so) had a 40MB RLL drive. Laptop form-factor systems were also relatively recent, with the floppy-based Toshiba T1100 being released in 1985 and the hard-disk enabled T1200 in 1987. Obiviously desktop drives of the era weren't ideally suited to laptop use, at that time 5.25" half-height was the dominant form-factor. The ST-506 interface also required two separate cables connecting the drive to the controller: a 34-pin control cable on which drives could be daisychained, and a 20-pin data cable from each drive directly back to the controller. These connectors alone take up almost 3.5" of space along the edge of the HDD logic board, and that still leaves power, which makes developing an alternate interface fairly logical.

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Getting things running

As I mentioned in my previous post, my friend Chris over in the UK was able to aquire a Victor V86P laptop, and very generously sent it on to me. The V86P is an IBM PC XT compatible laptop, released in 1989 by Victor Technologies. Quite a neat little unit, they had the following specs:

  • 80C86 CPU running at 10MHz (with 4.77MHz available as a keyboard shortcut)
  • 512k or 1MB RAM
  • Either 1x 720k FDD and 1x 20MB HDD
    Or 2x 720k FDDs
  • CGA graphics, capable of driving either the LCD panel or an external display
  • Monochrome LCD display (no backlight)
  • 1800mAh 6V NiCd battery pack
  • CMOS BIOS configuration storage, backed by supercapacitor
  • Real Time Clock
  • 1x Parallel Port
  • 2x Serial Ports (16450 UART)
  • Expansion bus connector
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