4 ms·
It's a bit tricky. From the 8008's perspective, it's running on -14 volts. But the 8008's Vcc (i.e ground) is connected to +5V, and the Vdd is connected to -9V.
by kens 6y ago
It's a bit tricky. From the 8008's perspective, it's running on -14 volts. But the 8008's Vcc (i.e ground) is connected to +5V, and the Vdd is connected to -9V. So when the 8008 outputs a 1 (high), the output is +5V (more or less), which is TTL-compatible. When the 8008 outputs a 0 (low), the output will be negative. Apparently, TTL is fine with negative input voltages, so this is acceptable.
The point is by shifting the 8008's supply to +5V/-9V and ignoring ground, the outputs automatically work with TTL.
- egsmi 6y agoInteresting, I thought TTL didn’t like voltages that were too negative but I’m definitely not too great on the historical stuff. This data sheet for 7400 logic, which admittedly only goes back to 1983, doesn’t like inputs below -1.5V That could very likely be because ESD protection was added and 1970 TTL parts didn’t have that. https://www.jameco.com/jameco/products/prodds/910901.pdf https://www.jameco.com/jameco/products/prodds/910901.pdf
- userbinator 6y agoDue to the threshold voltage, the low will be quite close to 0V. A TTL input is the emitter of a transistor, so trying to pull it slightly negative won't do any harm; and if anything, may actually help the transition be a bit sharper: https://en.wikipedia.org/wiki/Transistor%E2%80%93transistor_logic#Fundamental_TTL_gate https://en.wikipedia.org/wiki/Transistor%E2%80%93transistor_...
- egsmi 6y agoI'm a little slow today. There was an article about a month ago on the Sac State 8008 and it had a link to the user manual which contained a high level schematic of that computer. Sync goes straight into a TTL inverter. So ya, TTL, of 1972 vintage anyway, didn't seem to mind -9V as an input. https://news.ycombinator.com/item?id=24515156 https://news.ycombinator.com/item?id=24515156 Figure 10 shows how the interfacing of that machine worked. http://www.digibarn.com/stories/bill-pentz-story/docs/8008UM.pdf http://www.digibarn.com/stories/bill-pentz-story/docs/8008UM...