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The Tei Processor Terminal PT. 2

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So, a lot has happened since my last post (ah the perks of working part time), when I left off all the cards where out of the system and I had just finished sorting out the damaged power supply. One of my cards had a dead short on it between the -16v rail and ground but aside from that everything was pretty much unknown to me.  The first order of business, before I even thought about applying power to any of the cards (although the seller had already done that helpfully, grrr) was to backup any irreplaceable information. First port of call was the 2758 in the CRT controller. The 2758 is an interesting mongrel chip, being one of the first, if not the first single supply EPROM. It's effectively a 2708 that only needs 5v. If you want to read a little more about it, I enjoyed this article. Now my particular one was presumably being used for character generation, or perhaps setting up the CRT5027. As my crt controller is so rare I was keen to get this backed up, as a 5v EPROM the proc...
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Digging in to the Tei Processor Terminal PT. 1

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  So recently I managed to acquire myself an S-100 system for relatively cheap (but still fairly pricey to me). Now I've wanted an S-100 system for a very long time, but due to their cost and most of them being located in America, I considered this a pipe dream. So of course when one became available to me I snapped it right up without any consideration, consider it an early christmas present.  So what did I acquire? A Tei Processor Terminal (either 208 or 408, the jury is still out) all the way from Houston in Texas, how did it get across the pond? Who knows! It has been converted to run on 220v and seems to work fine on 240v so there must have been someone importing these.  If you search for information on this computer you'll probably find that image being used, and for good reason, it's one of very few that exist. It turns out Tei, although seemingly fairly well regarded in the industry at the time, is relatively obscure today. Although they talked about being founded...
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Leningrad construction Project Pt 2: hot swapping ROMS

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 So at the end of the previous update one of the ROM sockets was modified to accept a 27c128 (instead of using two 27C64 as is standard) of course I didn't have any 27c128 so I ended up using a 27C256 32KB EPROM with the image of the operating system copied twice over (N.B this wasn't actually needed. I could've just burned it into the upper half). In my search for the stock ROM however I found this site . An archive of various ZX Spectrum ROMs from various countries and different machines. So for reasons I will disclose in the future I have a bundle of 27C512s lying around, this got me thinking. There's a few different ROMS that caught my eyes when I was browsing so what if I could swap between them without having to go through the process of removing and replacing the EPROM. Now of course I could just put a dip switch on the extra address lines and be done with it but that might lead to crashes and instability if say, I flipped the switches whilst the computer was ru...
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Leningrad construction Project Pt 1: The bare minimum.

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So what is a Lenigrad? Well seeing as you found this post I presume you might already know, but for those of you who don't, the Leningrad-1 (Ленинград-1) (A.k.a Leningrad48, Zonov Variant) was one of the most popular clones of the ZX Spectrum in the former soviet union. It was designed by Sergei Zonov in 1988 and of course does not use ULA but instead uses integrated circuits of the к555 family (equivalent to 74LS) with a к561ие10 (MC14520) thrown in for good luck. It's a fairly elegant design (if you're into that sort of thing) as it does so much with so little. The best schematic ive found is located here ( tranlated ) or here ( translated ). Of course it has its own caveats and compromises (perfect is the enemy of good after all) but we will get to them in time.  So as I mentioned in the previous post, I began this project and abandoned it a few years ago, only having rebegun it recently. The state I left it in was as such: I put hours into soldering it and made a cu...
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