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Old 12-10-2007, 06:30 PM
Cappellanus Cappellanus is offline
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Join Date: Jul 2007
Posts: 24
Default more comments (ttl, etc.)

I'll attach a picture of the inside, but not much to show. This looks just like your machine. I tried to neaten up the wiring by twisting the wires a bit, but it didn't do much. Maybe a nice braid next time? Still, the original machines had individual wires to the front panel - that made quite a jungle.

Regarding connecting TTL inputs to VCC through a resistor:

While I thought it was crazy to tie TTL inputs to VCC through a resistor, apparently some early TTL chips needed this. I found internet posts suggesting "antique" 7400 series TTL with "multiemiter input transistors" sometimes needed this. Apparently, power supply noise or transcients can allow an input to be 1/2 volt over VCC, and cause chip failure. Any data sheet stating max input of 5.5 volts were susceptable to this. 74LS and some other chips were immune because of slightly different inputs. I wondered why I hadn't heard of this before. I managed to look at a copy of a 1971 TTL data book here...
Looking on page 6-4, it gives 3 options for unused inputs of AND/NAND gates. You could tie them to an independent power supply of 2.4-3.5 volts, or tie them to another input, or pull them up to VCC through a 1K resistor. It seems to only mention NAND/AND gates, which use the "dual-emitters" so I'm not certain it's needed on the XOR gate or set/reset lines of the 7474's as the Kenbak-1 uses. None the less, there *is* a reason for what John did.

Regarding decoupling capacitors:

I seem to remember recommendations of a decoupling capacitor, 0.01 to 0.1 uF, one for every 4-5 7400 TTL chips. Interesting to note that Intel recommended 41 separate decoupling capacitors on a single Pentium II (all around the outline) and faster processors really need distributed capactor PC boards, where power and ground planes exist in the same layer, separated by a dielectric, which acts as a capacitor along the whole board. I'm surprised that the 131 -chip Kenbak-1 works so well with zero decoupling capacitors, just a couple big electrolytes.

Regarding heat of the machine:

I agree, my machine really doesn't get too warm. A couple chips and the memory gets hot, but the case doesn't get warm at all. Not sure why the older Kenbaks needed a fan and holes in the top of the case. My Mark-8 makes lots of heat, but that seems to be from the linear power supply. It's conceivable that the linear power supply in the original Kenbak-1 dissipated twice as much heat as the entire board. Also, I suspect the more vintage 7400 TTLs dissipated more heat than newer ones. Another theory I don't have time to test.
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Last edited by Grant Stockly; 09-10-2012 at 09:15 PM.
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