View Full Version : Fairchild F8 Kit 1 Back in Operation

April 23rd, 2018, 05:24 PM
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:

This thread from 2011: http://www.vcfed.org/forum/showthread.php?25176
Fairchild F8 information on BitSavers: http://www.bitsavers.org/components/fairchild/f8/
Veras Systems F8 Kit manual on archive.org: https://archive.org/details/FairchildF8DevelopmenKitAssemblyManual

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:

http://i.imgur.com/QIe12T9h.jpg (https://imgur.com/QIe12T9)

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

http://i.imgur.com/a5YAGoxh.jpg (https://imgur.com/a5YAGox)

Before replacing the SRAM with sockets:

http://i.imgur.com/uJp7qsGh.jpg (https://imgur.com/uJp7qsG)

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.

May 6th, 2018, 10:00 AM
Just picked up the same F8 board and a set of documents this past weekend. Now to put together a current loop adapter.

- Gary

May 6th, 2018, 10:18 AM
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.

May 9th, 2018, 05:52 AM
I like that power supply. :)

The running F8 is nice, also.

Mike Willegal

May 9th, 2018, 06:23 AM
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.

July 1st, 2018, 04:51 PM
Writeup finished! http://www.glitchwrks.com/2018/07/01/fairchild-f8-kit

July 3rd, 2018, 06:46 PM
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!

July 4th, 2018, 04:41 AM
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.