@DouginRB....I'd love to be in So Cal having a Barbie, but I'm in Australia - a place called Bathurst inland from Sydney, It was minus 3.4 degrees C yesterday and probably about minus 2 this morning - we are in the middle of our winter. Not Barbeque weather at all..... but have a great Independence day and enjoy your BBQ!
PS I come from Rhodesia which is probably the only country apart from yours that declared independence from Britain. Ours only lasted 15 years, so you guys have done a bit better - well done!
@DouginRB.... are you aware you can change / correct your comment posts using the Edit/Delete link below your post (you have to be signed in)? However do NOT use it to get rid of great puns like that - there are a lot of us here who appreciate them!
Compelled to point out that the board pictured isn't a Nova 1200 CPU; it's too modern with the 40 and 22 pin chips! Here's a photo of an actual Nova 1200 CPU: http://imgur.com/IlWE6sK . Designed in 1970 or so, this particular board was manufactured in 1976.
The 74181 chip is top-center; the other 24pin chips are Signetics 8264 muxes to feed it. It was a brilliant design for its day. Nibble-wide processing meshed perfectly with the many 4-bit wide TTL chips available (7489 16x4 RAM, 74170 4x4 RAM, etc), and there was little speed penalty since the core cycle time of 1200ns still dominated the instruction cycle time of 1350ns.
The asymettry persists to this day in CMOS MCU's , typically a 4:1 or 10:1 asymettry on the pins when configured as outputs. This has a lot of advantages:
Much more common to short a signal wire to ground (harmless with soft pull up)
You don't have to run 5v to your switches (and risk shorting that to ground)
You bolt together 5V CPU's (with a 1.6v threshold) with 3.3v CPU's (with 1.6v threshold)
You can drive a LED to gnd directly from a CPU pin without a resistor
Less of an issue connecting together two devices, where the 5V on each may not always be present
I'm a big fan of active low too! , this still persists other than in ULN2003, e.g. very rarely does one see PMOSFETs used in 100v + power applications, so you are generally stuck with NMOS switch to gnd circuits.
Texas Instruments was not the earliest semiconductor company to promote the use of multiple emitter transistor coupled coupled logic circuits. A small semiconductor company in Calif. called Pacific Semiconductors Inc.(PSI)1st developed transistor squared logic (T2L)circuits in early 1961.The company competed with T.I. for an Air Force contract for the tri-service (TFX) fighter electronics. T.I. won, and PSI was subsequently aquired by Thomson-Wollridge (TRW).
The resistor coupled logic (RTL)circuits had a noise immunity problem with the "bottle geometry" if the base resistor was integratef with the transistor.Tsquared logic circuits had a high transistor leakage characteristic because of the "inverse beta" of the coupling transistor unless special processing was used in the construction of these devices.
Don Schulz P.E. (retired)
ex PSI, TRW employee.
Sheetal wrote: PCBs just used to be so crowded with these small components and how difficult it was to do troubleshooting.
Nowadays PCBs are crowded with even smaller components in 0.4mm pitch BGAs and QFNs. For debugging, it was sure was nice to be able to attach DIP clips and hook up 'scope probes to those 0.1" pitch pins...