4 ms·

The Cray-1 used bipolar ECL (emitter-coupled logic), which was very fast but power hungry. Thus, the Cray-1 needed to be immersed in Freon for cooling. You can

by kens 7y ago

The Cray-1 used bipolar ECL (emitter-coupled logic), which was very fast but power hungry. Thus, the Cray-1 needed to be immersed in Freon for cooling. You can think of ECL as differential amplifiers that steer current down one path or another. RTL (resistor-transistor logic) is entirely different. It was used in the Apollo Guidance Computer because that's all integrated circuits gave you at the time, but it's not very good. (The inputs affect each other because they are only separated by resistors. Even diode-transistor logic is much better.)

TheOtherHobbes 7y ago
ECL is fast but hot because the transistors are never fully saturated, so current is always flowing. CMOS is more like the electronic equivalent of a stiff mechanical lever system. Logic voltages change the charge distribution inside the MOS transistors. There's almost no current flow outside of them. This is great for super-low power draw, but it's not particularly snappy, it can't drive LEDs directly, and it's not happy driving long wires without buffering. It's also very easily damaged by static electricity - which is why wrist grounding straps are a thing. Because the transistors are so tiny it's easy to scale it to VLSI. TTL is mid-way between the two. The transistors saturate cleanly, so switching is faster than CMOS. The source/sink currents are much smaller than with ECL, so power requirements are manageable, although switching glitches need good decoupling. It's a robust and reliable system, with low-power options. But it's not as dense as CMOS, which is why 74xx series TTL never got any more complex than a simple 4-bit ALU, while MOS soon expanded into full 8-bit VLSI microprocessors.
- blattimwind 7y ago
  > but it's not particularly snappy Fast enough for practically all processors made today > it can't drive LEDs directly That depends on the design, but CMOS chips generally have much better symmetric output characteristics than TTL ever had. Most MCUs can drive quite some current on the output pins, and 74HC and similar logic families can certainly drive LEDs.

by kens 7y ago

TheOtherHobbes 7y ago

ECL is fast but hot because the transistors are never fully saturated, so current is always flowing. CMOS is more like the electronic equivalent of a stiff mechanical lever system. Logic voltages change the charge distribution inside the MOS transistors. There's almost no current flow outside of them. This is great for super-low power draw, but it's not particularly snappy, it can't drive LEDs directly, and it's not happy driving long wires without buffering. It's also very easily damaged by static electricity - which is why wrist grounding straps are a thing. Because the transistors are so tiny it's easy to scale it to VLSI. TTL is mid-way between the two. The transistors saturate cleanly, so switching is faster than CMOS. The source/sink currents are much smaller than with ECL, so power requirements are manageable, although switching glitches need good decoupling. It's a robust and reliable system, with low-power options. But it's not as dense as CMOS, which is why 74xx series TTL never got any more complex than a simple 4-bit ALU, while MOS soon expanded into full 8-bit VLSI microprocessors.

blattimwind 7y ago
> but it's not particularly snappy Fast enough for practically all processors made today > it can't drive LEDs directly That depends on the design, but CMOS chips generally have much better symmetric output characteristics than TTL ever had. Most MCUs can drive quite some current on the output pins, and 74HC and similar logic families can certainly drive LEDs.