Electronics > Beginners
Whats the purpose of using 3 transistors in an AND gate?
amyk:
More famously, the Apollo Guidance Computer was entirely constructed of 3-input RTL NOR gates:
http://klabs.org/history/ech/agc_schematics/logic/5011-1.jpg
http://klabs.org/history/ech/agc_schematics/
rhodges:
Thanks, that schematic is so easy to read. When I got my TI TTL databook almost four decades ago, I read the logic just fine, but the transistor diagrams made my eyes glaze over. I wonder if I still have that book somewhere...
It would be fun to read what resistor values were used in these RTL circuits, the expected hfe values, and the fan-outs. Could be a good story there.
David Hess:
Fairchild included complete schematics with resistor values in their RTL datasheets. They show base series resistors of 260 ohms and collector load resistors of 640 ohms.
Modern TTL and DTL has much better noise immunity through the use of additional diode junctions than RTL and early DTL. The early stuff was actually pretty horrible.
http://semiconductormuseum.com/MuseumStore/MuseumStore_Fairchild_923_Index.htm
rstofer:
We got to the Moon with RTL. Pretty impressive.
Diode logic was prett cool. 1N914s everywhere!
David Hess:
--- Quote from: rstofer on August 21, 2019, 01:45:43 am ---We got to the Moon with RTL. Pretty impressive.
Diode logic was prett cool. 1N914s everywhere!
--- End quote ---
For the moon, I think it just came down to hybrid construction where screen printed thick film resistors are essentially free but every transistor or diode was an extra part to be wire bonded.
TTL eventually went back to using diode input structures but devices on complementary bipolar processes used PNP emitter followers.
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