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GeneralRe: LOL Pin
Brisingr Aerowing11-Jan-13 13:59
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GeneralNuget Pin
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GeneralToday I embarked... Pin
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CDP180212-Jan-13 1:20
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Both, the signal diodes and the 74LS154 are alive and well. They would just not quite do the job.

Here[^] is a link to the article about building a little computer from 1977. If you look closely, you will see that it's completely CMOS, including the processor. Very unusual at that time, as well as building a simple complete little computer for under 100$.

People still build this computer today and there are also some modernized incarnations. A common 'feature' usually are IC8 and IC9 with the eight toggle switches to input data. My old computer was a slightly improved version where the eight switches were replaced by a 74C922 keyboard encoder and a 74C179 latch.

The encoder's output was only 4 bits wide, the value of the last key that was pressed. The latch stored the other four bits, so that you had a complete byte that could be read by those 4016 switches. A 1 to 16 decoder like the 74LS154 would not help very much here.

The 74C922 is out of production for many years. With some effort you may locate one, but then it will be relatively expensive. What we need is a replacement that delivers a full byte, the last two keys that have been pressed. If possible, with standard logic ICs. God forbid, no microcontrollers or programmable logic. It should be kept simple, cheap and easy to reproduce. I needed parts for less than 15$, including the buttons for the keyboard.

For building a prototype on the breadboard LS TTL was convenient, but in the end I would certainly use CMOS again, replacing 74LSXX with 74HCXX or 74HCTXX. Mixing logic families usually is not a good idea and it would keep the little computer's power requirements low. There are some versions that can run for months on ordinary batteries.
GeneralWhat Linux is good for ? Pin
BillWoodruff11-Jan-13 13:22
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Brisingr Aerowing11-Jan-13 13:37
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GeneralA praise to the crowd! Pin
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Garth J Lancaster11-Jan-13 13:12
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GeneralWTE??? Pin
gavindon11-Jan-13 11:30
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GeneralRe: WTE??? Pin
Dave Kreskowiak11-Jan-13 11:36
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GeneralRe: WTE??? Pin
CDP180211-Jan-13 11:46
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GeneralRe: WTE??? Pin
Garth J Lancaster11-Jan-13 11:47
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GeneralRe: WTE??? Pin
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GeneralRe: WTE??? Pin
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