Comment by userbinator | Hacker News Reader

Comment by userbinator | Hacker News Reader

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To understand how the inverter works, when the input (pin 2) is high, the current through the tube pulls the plate (pin 1) low. Conversely, when the input is low, the electron flow is blocked and the resistors pull the plate high.

Note that "high" and "low" here are quite different from the usual digital logic conventions of high=+V, low=GND; in this case, high would be near GND while low is a negative voltage. The circuit manual PDF gives low=-30 and high=+10. A lot of other early transistor logic families operated with such "unusual" (for today) signal levels too.

The other notable characteristic of tube circuits is the high resistor values --- this is because they operate at low current (in the mA range) but high voltage (hundreds of voltages). Contrast this with transistor logic which is relatively high current and low voltage.

One amusing component I found in the tube module was "Vitamin Q" capacitors.

Those are actually very desired by audiophiles, so it's odd to see them in a digital circuit. They're paper-in-oil capacitors and "Vitamin Q" was Sprague's trade name for the proprietary oil they used.

A lot of other early transistor logic families operated with such "unusual" (for today) signal levels too.

One wacky thing in IBM's transistor computers is that they would alternate NPN and PNP gates. This avoided an extra transistor in each gate to shift the voltage level back. So you'd end up with one gate using +12V/0V logic levels and the next one using +6V/-6V.

This technique was used in three different transistor logic families, each with their own voltages. So there were 6 different transistor logic levels, plus other miscellaneous logic levels (e.g. 48V relays). IBM's old computers were a crazy collection of different voltage levels.

> transistor logic which is relatively high current

Huh? The gate resistance of a FET is hundreds of megaohms, so the input current is measured in nanoamps. That doesn't seem like high current to me.

That's the static resistance, but when it's switching there is current to charge/discharge the gate. Don't forget leakage currents too, which are quite high in modern CPUs --- they consume ~100A at ~1V when in active use.

Sure, but that is amortized over billions of transistors. If you had billions of vacuum tubes the current draw would be vastly higher.

Transistors in TTL logic use mA of input current because they're bipolar, not FET transistors.

In classic TTL the input currents are also very asymmetric, e.g. for a high level you mustn't draw more than a few dozen µA out of an input, for a low level you have to draw something like 1.5 mA or so.

This is different from current-steering based logic (all kinds of ECL, including CML/SCL) were the current in the circuit stays the same, but only takes a different path depending on state. Supply current is largely independent of circuit state with these.

That's true, but we're talking about computers here. Computer chips don't use TTL, they're all CMOS, which uses FETs (MOSFETs to be precise).

CMOS didn't exist when the first transistorized computers were made.

Ah, right. I thought the OP was referring to modern CPUs.

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