learning the basics

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Joined: Apr 16 2020 - 09:13
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learning the basics

I understand how transistors works and how you can use them to build logic gates. I also understand how a 6502 processor works, I mean I know how to make instructions using Assembler.

What I can’t imagine is how, using those resources, you can show an ASCII character in a screen. I mean how Wozniak start from logic gates and he could print some character in a TV set?? Seems like magic…

Does anyone could guide me how to understand it?

Last seen: 2 hours 3 min ago
Joined: Jun 18 2010 - 13:54
Posts: 705
Everything you want to know

Everything you want to know is in "Understanding the Apple II" by Jim Sather. Google it to find a downloadable pdf.

Last seen: 10 hours 17 min ago
Joined: Apr 1 2020 - 16:46
Posts: 717
If you really want to go to the root ...

... of how "Wozniak could print some characters in a TV Set", don't start with the Apple-II as suggested in post #2, especially if you are interested in the Apple-1 video system, which seems to be the case as the first request was not asked in the Apple-II forum, but on the Apple-1 forum.


I suggest you should first look into electronics guru Don Lancaster's "TV Typewriter Cookbook" or his TV typewriter article that appeared in the September 1973 issue of "Radio-Electronics Magazine". The former is available for free from the ebook section on Don's homepage:




For the magazine article there are plenty of scans floating around in the web. You just need to know the magic spell "Don Lancaster TV Typewriter" for any search engine, and Sesame will open for you with all the treasures within up for grabs.


Note that I don't intend to suggest that Woz took the terminal section circuit from Don Lancaster, as some nasty people have claimed in the past. IMHO this allegation is a long debunked myth, because it does not hold up to closer scrutiny. For instance, the counter chain in the Apple-1 terminal section shows the typical tricky, gate count saving techniques Woz is famous for, which he also carried over to the Apple-II. These techniques using the "load", "count enable" and "carry in/out" features of certain TTL counters in a mindboggling way are typical for Woz and not to be found anywhere in the work of Don Lancaster. IMHO, Woz probably started with a clean sheet design, supported by Signetics datasheets and their application notes for the 2504, 2513, and 2519 ICs, which became the core of the Apple-1. The Signetics application notes of the time showed block diagrams for text video displays based on this chipset. So the idea to use serial shift registers as a text screen memory was readily around, and probably a patent minefield, which would explain why Signetics showed only block diagrams, and not completely worked out circuits with all the details.


Still, Don Lancaster certainly is the pioneer who made "TV typewriting" accessible to the masses. He went on and pioneered the generation of video by software, i.e. on the KIM-1, as seen in his "Cheap Video" books. Clive Sinclair probably stole that idea from Don and twisted it a bit to make his infamous ZX-80, which IMHO was a blatant customer ripoff for its usurious price. Don Lancaster also wrote a lot of other great books on electronics, such as the "Active Filter Cookbook", which I used to my advantage throughout my professional career, and I still use it. And the best of all, Don is still around and among us !


A little bit of historical context may be of interest to readers who have been born too late to witness the enormous progress of electronics that happened in the 1970s:


In the early 1970s I was a teenager and already proficient as a neighborhood TV repair"man", which at the time involved looking up the symptoms in a little book full of faulty screen pictures and then replace the tube(s) listed under that picture, done. If some other component had died, it typically could be found by sense of smell or because it had fried itself and the PCB underneath, or had exploded. This is what got me into electronics at a tender age, and I witnessed the invention and rise of the microprocessor firsthand.


One interesting fact about this decade is that the small electronics hobbyist crowd quickly jumped on the digital bandwagon once TTL ICs became cheap enough and available for hobbyists in the early 1970s (plain vanilla TTL was "born" in 1963 and already on the way out, to be replaced by LSTTL and CMOS). Many a hobbyist tried to build a digital clock based on TTL and the yet new LED-based 7-segment displays. These digital clocks still were "magical" devices because none were for sale in the consumer market yet ! So there is no wonder that once the first "TV typewriters" were seen in magazine pages, they were even more magical and desirable than digital clocks ! And many hobbyists who could afford the ICs tried to build one ! Despite there was no microprocessor available yet ! So the TV Typewriters had no "brain" and all you could do is to type text and watch it appear in the TV. Duh ! Not very useful, but very, very magical moments with that, I can tell you !

There was only one other digital gadget for TVs which drove people crazy in the early 1970s, and this were the various incarnations of "Pong", or "TV Tennis". Which could be built from discrete transistors, 555 timers, and the circuits gradually were more and more digitized. Electronic dice based on 7 or 2 x 7 LEDs also were popular DIY gadgets. And the occasional digitized door chime. Or code locks. But that was about all a digital hobbyist could build before the microprocessor.  The various, quite popular electronics magazines were full of endless variations of these digital gadgets year after year after year. Of course they also featured lots of non-digital projects of all sorts, such as Hifi amplifiers, DIY radios, or boiled egg timers based on the 555, but in the end you could buy most of those gadgets in any well stocked department store much cheaper than if you built it yourself, and these consumer products also had a much nicer, "tailor" made case or enclosure.


The first somewhat useful microprocessor was the Intel 8080 which came out in April 1974, but it was heinously expensive: for the price of a new midline family car made in Detroit you could get only 25-30 of those 8080 ! Which needed three supply voltages (+12V, +5V, and -5V) and two support chips, the 8224 and 8228, to make a complete CPU. Unaffordable for most hobbyists ! In mid-1975 the first 6502 wafers came out of the fab and it was a game changer. For the price of one 8080 you could get eight 6502 which was faster and needed only a single +5V supply. As the story is usually told, Woz bought a few of the first publicly available 6502, still with the ROR bug, right out of the jar from which MOS Technology (according to some sources at the time already teetering on the brink of bankruptcy because the prices for its calculator ICs had collapsed), sold them during the Wescon trade show in San Fransicsco beginning on September 16, 1975.


The rest is history !            


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