What is TTL and why is it important to my Retro Computers

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What is TTL and why is it important to my Retro Computers

Post by rubber_jonnie » Wed Oct 06, 2021 11:20 am

TTL, or Transistor Transistor Logic (REF:https://en.wikipedia.org/wiki/Transisto ... stor_logic is the core technology used in the chips used within many of our favourite retro computers

In basic terms, TTL translates voltage levels into binary, 5V being 1 and 0V being 0.

It isn't quite as simple as that though, as the 5V & 0V scenario is what would be the case in a perfect world!

In reality the binary 0 is anywhere between 0V and 0.8V for an input signal and 0V to 0.5V for an output signal.

Binary 1 is anywhere between 2V to 5V for an input signal, and 2.7V to 5V for an output signal.

And of course the ICs need a power connection, or VCC of 4.75V to 5.25V

Why do we need to know this?

When investigating problems with these systems, it is often the case we will require a way to determine if these signals are correct.

For example, we want to check the output of an inverter, one of the simplest logic gates, which will output a TTL value of 1 when a signal of 0 is input, and vice versa.

We can use a TTL logic probe, however in a running machine, the input/output signals may be switching very quickly so the logic probe might not be able to give an accurate yes/no answer, though it will at least show the state is changing.

However, if we connect channel one of an oscilloscope to the inverter input, and channel two to the output, we can capture these changes of state and measure the voltages at the same time.

This allows us to see if the input/output voltages fall within the above specifications and if there is any delay in the output once an input is received. We can check this against the specs for a chip in the datasheet.

Similarly, we can use a Logic Analyser to collect similar data, though my preference is the oscilloscope.

REFERENCES:https://www.allaboutcircuits.com/textbo ... 20exactly.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...

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