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Thread: Fairchild F8 Kit 1 Back in Operation

  1. #1
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    Default Fairchild F8 Kit 1 Back in Operation

    album: https://i.imgur.com/uJp7qsG.jpg

    I'll do a full write-up on this one, but I just wanted to share that I got my F8 Kit up and going! This one came in a heap of scrap, the switches and some capacitors were busted off, someone had stolen then 3850 CPU and 3853 SRAM interface. Fortunately the 3851A PSU with FAIR-BUG was still in its socket -- this is like the 6530 on a KIM-1, it contains mask programmed code and there's no direct replacement. Anyway, I had some time today and decided to clean it up and get it working. I found the following resources very helpful:



    I happened to have a 3850 CPU from the display panel of a piece of old avionics gear, and I bought the 3853 SRAM interface from one of my regular suppliers. I didn't have the manual at first, just the BitSavers schematic, and missed the part about the current loop interface needing to be connected to some of the I/O port pins on the 22/44 connector, but could tell that it was bootstrapping properly from examining the address lines. Once I figured out that I needed to connect pin 10 -> 14 and pin 12 -> 15 on the 22/44 connector, I still had issues with current loop as I'd misread the PRINTER OUT designation as 3 instead of M! No harm done, fortunately I got side-tracked on that and interfaced a MAX232 to the TTL serial lines on the edge connector, but the board was behaving strangely. I suspected bad RAM, so I desoldered all eight 2102s and replaced with sockets. No improvement. I then took another look at the current loop generator and discovered...the data was inverted! It was at that point that I realized my 3 vs. M designation error, so I switched back to current loop...and...success!

    I was using my VT220 in current loop mode at 110 baud for these tests. For that, you want to connect:

    F8 Kit pin 1 -> VT220 pin 5
    F8 Kit pin 2 -> VT220 pin 3
    F8 Kit pin 13 -> VT220 pin 2
    F8 Kit pin M -> VT220 pin 7

    Hooked up to my bench power supply:



    Editing address 0x0000 using FAIR-BUG on the VT220:



    Before replacing the SRAM with sockets:



    Interesting tip for debugging: none of the RAM needs to even be present for the FAIR-BUG monitor to work. Apparently it uses the scratchpad registers exclusively. So, don't go chasing RAM issues if FAIR-BUG doesn't come up! My oscillator was way out of adjustment when I got the board up and going, so I ended up twiddling the adjustment trimpot while punching reset, looking for the question mark prompt to come up. I found both extremes and picked an adjustment in the middle. I haven't verified frequency of the main oscillator yet.

  2. #2
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    Just picked up the same F8 board and a set of documents this past weekend. Now to put together a current loop adapter.

    - Gary

  3. #3
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    Quote Originally Posted by gekaufman View Post
    Just picked up the same F8 board and a set of documents this past weekend. Now to put together a current loop adapter.

    - Gary
    There's TTL serial available on the 22/44 connector, but you do need to invert at least the transmitted-from-F8 line. Also there are bits on the connector that let you select between 110 and 300 bps, so if you strap it for 300 bps and run a MAX232 or something off the TTL serial pins, you should be able to talk to it from a modern PC as long as the comms program supports going that slow. I've done 300 bps with Prolific type USB->RS232 adapters under Linux, no problem.

  4. Default

    I like that power supply.

    The running F8 is nice, also.

    Mike Willegal

  5. #5
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    Hah, thanks The old Lambda triple-voltage supply has served me well. I use it with a Morrow WunderBuss backplane for testing S-100 boards, as well -- the current limiter will usually stop tantalum caps from totally running away and exploding.

  6. #6
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  7. #7
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    Neat! I've been kind of looking for one of these for a while but they don't seem to show up much on ebay. I suspect a lot of would be sellers aren't really sure what they are. Nice work!

  8. #8
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    Thanks! I don't think I've ever seen one for sale on eBay. As mentioned, this one came in some industrial scrap I bought from one of my day-job clients. It's a simple enough board for one to build it up on perfboard. Weirdly enough, the FAIR-BUG PSUs do show up. I bought some spares from, IIRC, Unicorn Electronics, which worked fine.

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