Welcome to this deep dive. You know, this show
takes a stack of your sources, articles, research,
your own notes, and we just extract the most
important nuggets of knowledge. Yeah, think of
it as like your personal shortcut to being well
informed. Exactly. And today we have a custom
deep dive tailored specifically for you, the
listener. We are exploring a single, honestly
wildly fascinating Wikipedia article on something
called three state logic. It sounds super technical.
I know, but it's actually everywhere. Right.
And to set the stage, I want you to picture a
massive busy city intersection, like six lanes
of traffic in every single direction. OK, I'm
picturing it. Now imagine there is zero traffic
lights, no stop signs, and literally everyone
is driving at top speed. Oh, wow. I mean, that's
just total instantaneous chaos, wreckage everywhere.
Pure chaos. And that interception is exactly
what the inside of your computer or your smartphone
or even your digital microwave would look like
without this secret. hidden third state of digital
electronics. Because we've all been taught the
exact same thing, right? From the very first
time you take a computer class, you are fed a
very specific myth. Yeah, that computers run
entirely on binary. Just ones and zeros on and
off. Right. High voltage and low voltage. And
I mean, it is a very comforting, very straightforward
way to view the digital world. You either have
power or you don't. OK, let's unpack this. We
have our standard binary logic, a high voltage
output state, which represents a logical one.
Yeah. and a low outfit state representing a logical
zero. So what on earth is the third state? Because
you currently have half a voltage in pure digital
logic. No, you can't. In digital electronics,
this third state is actually called high impedance.
It's typically abbreviated in engineering as
high Z. High Z. OK. And the crucial thing to
understand right out of the gate is that high
Z isn't a voltage level at all. Wait, really?
The article uses that term, impedance. But let's
translate that into plain English for a second.
What is impedance physically doing? So think
of impedance simply as electrical resistance.
When a device enters the high impedance state,
or high Z, it's essentially throwing up a massive
impenetrable wall of resistance. Oh, I see. The
resistance is so incredibly high that electricity
simply cannot flow through the gate. The output
of that component is effectively like physically
disconnected from the rest of the circuit. So
it behaves exactly like an open switch. Exactly
like an open switch. So if a logical one is say
someone standing in a room shouting and a logical
zero is someone in that same room whispering.
Hi -Z isn't just staying quiet. Hi -Z is leaving
the room entirely, locking the door, and basically
ceasing to exist as far as the room is concerned.
What's fascinating here is that analogy perfectly
captures the physics of the situation. It's the
absolute removal of a device's electrical influence
from the shared circuit. A state of total disconnection.
Yeah, intentional disconnection. And to understand
how engineers actually apply this, your source
material points us to something called a tri
-state buffer. Right, I saw that. The article
details the truth table for this buffer. It's
a little gateway that relies on an enable signal
to actually function. Right, because a standard
buffer just takes an incoming signal and passes
it directly to the output. Just a straight pass
through. Yeah. But a tri -state buffer has an
extra pin. this enable signal you just mentioned.
When that enable signal is turned off, the buffer's
output slams right into that high Z state. It
literally vanishes from the circuit. It leaves
the room. It leaves the room. But when the enable
signal is turned on, the buffer wakes up and
acts like a regular gateway. It duplicates the
input, whether that's a one or a zero. and pushes
it through to the output. Now, I have to push
back on something here, because the text mentions
a secondary feature of this buffer that, honestly,
it confused me at first. Yeah, with the restoration
part. Yeah. It says that when this buffer is
enabled, it doesn't just lazily pass the one
or the zero along. It actually provides voltage
-level restoration. Yes. But why does a digital
signal need restoring? I mean, if I pass you
a perfectly good one, shouldn't you just pass
a one to the next component? Well, that is where
the pristine mathematical theory of ones and
zeros violently collides with the messy, harsh
reality of analog physics. Oh, right. Physics
ruins everything. Exactly. As an electrical pulse
travels through microscopic copper wires, across
solder joints and past other humming components,
it degrades. It gets tired. A crisp, sharp, 5
-volt high signal starts to sag and lose energy.
And a crisp, 0 -volt low signal might pick up
some ambient electromagnetic interference and
actually rise slightly. Oh, wow. So they start
drifting toward the middle. Exactly. And if you
don't actively clean that up, eventually your
1 sag so low and your 0 rises so high that the
receiving chip literally cannot tell them apart.
Ah, so the tri -state buffer, when it's in the
room and actively participating, is also acting
like a signal booster. Precisely. It takes a
tired, sagging, degraded signal, looks at it
and says, OK, you're at 3 .8 volts, you're clearly
supposed to be a 1, and it blasts out a fresh,
strong, perfectly valid 5 -volt signal to the
next phase. It guarantees the output is well
within the valid logic voltage range. Oh. And
for the sake of thoroughness, your sources also
note you can get inverting tri -state buffers.
Which do the same thing, but flip the signal.
Yeah, they do the exact same restorative boosting,
but intentionally flip a one to a zero, or vice
versa, depending on what the circuit actually
needs. OK, so what does this all mean for how
our devices actually function? Like, why is this
ability to completely vanish from the circuit
so critical? Well, the article immediately points
to something called shared buses. Right. Think
of a bus and a computer as a massive multi -lane
superhighway. of copper wires that connects everything
together. Like the motherboard. Exactly. Your
central processing unit, your system memory,
your graphics card, USB controllers, they were
all physically soldered onto the exact same set
of microscopic copper traces. So they literally
share the exact same physical wires, which instantly
brings us back to that chaotic intersection analogy,
right? Or maybe like a crowded telephone party
line where everyone is just screaming over each
other. But if they are all physically wired to
the exact same copper line, what literally stops
them from all trying to talk at once and blowing
a fuse? If we connect this to the bigger picture,
this is where hardware level diplomacy becomes
a matter of life and death for the machine. Life
and death. Literally. Imagine if we only had
our standard 1s and 0s with no high Z state.
Device A needs to send a 1, so it floods the
shared wire with high voltage. But at the exact
same millisecond, Device B decides to send a
0, so it aggressively pulls that exact same wire
down to ground or low voltage. So they're arm
wrestling over the exact same piece of copper.
And the loser of that arm wrestling match is
your computer. That direct fight between high
and low voltage is the textbook definition of
a short circuit. The high voltage flows unimpeded
directly into the low voltage pin. You get a
massive excessive current draw. And what happens
to the data? The actual data on the wire is completely
destroyed. The voltage just hovers somewhere
in the middle as pure useless noise. And those
tiny silken... pathways generate incredible instantaneous
heat. They literally burn up. They burn out completely.
And that is why we physically cannot build complex
modern printed circuit boards without the high
impedance state. So it's fundamentally necessary.
Absolutely. On a shared bus, every single device
sits securely behind a tri -state buffer. When
your computer's CPU needs to read a file from
memory, the memory chip enables its buffer, drives
the bus with its data. And everyone else gets
out of the way. Exactly. Every single other device
on that entire shared highway goes into high
Z mode. They all pull over, turn off their engines,
and leave the highway so the memory chip gets
a completely clear road. No crosstalk, no noise,
no short circuits. That's amazing. The source
highlights a phenomenal real -world example of
this too. The serial peripheral interface bus.
Or SPI. OK, how does SPI use it? In a multi -drop
SPI setup, you have one master controller chip
talking to multiple peripheral chips, all sharing
the same communication lines. Right. To prevent
that catastrophic short circuit, the master uses
a dedicated pin for each peripheral called chip
select, or CS. So it's like a teacher in a classroom.
The master chip taps one specific peripheral
chip on the shoulder using the CS pin and says,
you. Talk. Perfect analogy. And because all the
other chips in the room don't have their CS pin
activated, their outputs remain strictly in the
high Z state. They physically cannot interrupt.
It is brilliant hardware design. And this ability
to selectively vanish isn't just for massive
expensive computer motherboards either. It's
used everywhere. Like where else? Have you ever
looked at a tiny digital clock or a custom keyboard
and wondered how it can light up hundreds of
individual LEDs? Even though the cheap little
microchip controlling them only has a handful
of physical pins. Actually, yeah. I've always
assumed it was some sort of rapid -fire optical
illusion, like some multiplexing trick. You are
definitely on the right track. The source mentions
a specific technique called Charlie Plexing.
Charlie Plexing, okay. Imagine a grid of LEDs.
If you want to light up just one specific bulb
in the middle of the grid, you need to send a
high voltage to its row and a low voltage to
its column. Right, creating a path for the electricity
to flow through that one specific LED. But electricity
is notoriously lazy, and it will try to find
sneak paths through the other surrounding LEDs
in the grid, faintly lighting up bulbs you actually
want to remain dark. Oh, and in the light. Yeah.
So Charlie Plexing solves this by utilizing our
third state. The microchip rapidly switches the
pins controlling the other rows and columns into
the disconnected high Z state. Oh, wow. So by
going into high impedance, it essentially acts
like a pair of scissors, temporarily snipping
the wires of those alternate paths. Yes. The
electricity has literally nowhere else to go
except through the one specific LED you targeted.
It forces the current down the single correct
path. It's this incredibly fast microscopic dance
of connecting and disconnecting. And speaking
of that microscopic choreography, here's where
it gets really interesting in your source material.
Oh, I know where you're going with this. Yeah.
When we talk about memory chips like RAM or ROM
sharing these communication buses, the article
reveals a fascinating, deeply elegant trade -off
between speed and power consumption. The subtle
but vital difference between chip select and
output enable. Yes, because the article points
out that many memory devices have both a CS pin
chip select and an OE pin output enable. Right.
And if you just glance at the spec sheet, they
seem completely redundant. I mean, if either
of those pins is turned off, the chip's output
goes into that disconnected Hi -Z state. Why
do you need two different off switches? Because
how they achieve that disconnection internally
is radically different. It's honestly a master
class in engineering optimization. Let's break
it down for the listener. When chip When the
memory select is turned off, like when it is
de -azerted, the article says the chip basically
goes into a deep sleep. Right, it powers down
its internal circuitry. Which is fantastic for
battery life. Yeah, if a memory chip isn't currently
needed, you definitely want it asleep. But there
is a massive penalty for that power saving. About
a wake up time. Exactly. When you finally turn
that chip select back on, the chip has to wake
up, stabilize its internal power, receive the
address of the data you requested, physically
locate that data deep within its vast memory
array, and then route it all the way to the exit
gate. That's a lot of steps. It creates a huge
latency delay. OK, I want to try an analogy here
to see if I've got this. Is chip select like
turning your kitchen oven completely off to save
electricity? OK, yeah. You save power, but when
you want to bake a cake, you have to wait 20
minutes for the oven to preheat from cold. Right.
Whereas output enable is like, keeping the oven
on, fully baking the cake, but just keeping the
oven door closed and locked until the exact second
you are ready to serve it. That analogy perfectly
captures the mechanism. If you only rely on chip
select, you are forcing the computer to wait
for the oven to preheat every single time it
needs a piece of data. Which is super slow. But
if you assert chip select, meaning you turn the
chip fully on, but you keep the output enable
pin deserted, the chip is wide awake and working
furiously. It's baking the cake. It's baking
the cake. It receives the address from the CPU.
It finds the data. It routes that data all the
way to the final exit door. But because output
enable is off, that final door is locked in a
high impedance state. Exactly. The chip gets
everything 100 % prepped and just waits patiently
at the final driver. And here is the genius part.
What's that? While the memory chip is doing all
that internal searching, the main system bus
is totally free. Other components can be furiously
passing data back and forth across the motherboard.
Because our memory chip, despite working hard
internally, is still technically disconnected
from the shared highway. It's not blocking traffic.
And then the very millisecond the shared bus
is clear and the CPU is ready for the data, the
system asserts the output enable pin. The door
opens. the Hi -Z door vanishes, and the data
floods out onto the bus with practically zero
delay. In fact, if you look at a highly detailed
spec sheet for a ROM or static RAM chip, they
literally list two completely separate access
times to account for this. Let me guess. A painfully
long time for a chip select wakeup, and a dramatically
shorter time for an output enable door open.
You'll see times listed in tens of nanoseconds
for a full chip select cycle, compared to maybe
just a few nanoseconds for an output enable cycle.
That is such a clever way to cheat latency. But
wait, I have to stop and ask about something
that feels like a glaring flaw in this entire
architecture. OK, what is it? We firmly established
that high Z means disconnected. It's an open
switch. Yes. If you have a shared bus, and nobody
is currently selecting, let's say the CPU is
busy doing math internally, and every single
peripheral device has left the room and is sitting
in a high Z state, what physically happens to
the copper wire itself? Ah. It's just sitting
there attached to absolutely nothing. This raises
an important question. And it's a physical reality
that severely plagued early circuit designers.
When all the outputs attached to a bus are tristated,
the circuit node enters what we call a floating
state. Floating? That sounds... I'm assuming
floating is bad. Floating is a nightmare in digital
logic. It really has no defined voltage level
at all. An unattached copper wire acts exactly
like a tiny radio antenna. Oh, picking up interference.
Exactly. Ambient electromagnetic noise. From
the power supply, from a nearby cell phone, from
the literal fluorescent lights in the room, can
cause the voltage on that floating wire to drift
up and down randomly. And if a component suddenly
wakes up and samples that line while it's floating?
It might read a false one or a false zero, causing
a total system crash. So how do you anchor a
floating wire? I mean, you have to stabilize
it somehow. Engineers fix it with something incredibly
simple. Resistors. Specifically, pull up or pull
down resistors. Usually somewhere in the range
of 1 to 100 kilo ohms. Wait, hold on. If the
line is literally just floating there disconnected,
how does adding a resistor, which is, you know,
literally a roadblock for electricity, magically
create a stable signal? It seems counterintuitive.
Yeah. Aren't you just making it harder for the
electricity to flow? Let's visualize it. You
take the shared bus wire and you tie it directly
to your main high voltage power source. But you
put that 10 kilo ohm resistor right in between
them. OK, I'm visualizing it. If any chip on
the bus actively wants to drive the line low
to send a zero, its internal transistors are
strong enough to easily pull the voltage down,
completely overpowering the resistor. Because
the resistor is weak. Right. But when everyone
goes into high Z and lets go of the line, that
gentle pull -up resistor acts like a weak spring.
It slowly, safely pulls the voltage of the floating
wire up to a default logical one. So it's literally
like a weak mechanical spring that always pulls
a door shut when nobody is actively holding it
open. It guarantees the door is never just like
flapping in the wind. That's the exact mechanism.
But as with all things in engineering, solving
one problem immediately creates a new, more complicated
one. Of course it does. As computers got faster,
specifically when the industry moved to things
like the PCI local bus in the 1990s, this weak
spring solution simply wasn't fast enough. I
noticed that. The sources mentioned the PCI bus
having a large distributed capacitance. Yes.
What does that mean in plain English, and why
does it break our weak spring? Well, capacitance
is essentially the ability of a physical component
to store an electrical charge. A long copper
wire running across a motherboard with dozens
of chips attached to it acts like a tiny unintentional
battery. Oh I see. It actually takes time to
fill that wire up with voltage and it takes time
to drain it. So if you just rely on a weak 10
kilo ohm resistor to pull the voltage up the
battery of the long wire just absorbs all that
weak energy. It's like trying to fill a swimming
pool with a dripping faucet. That would take
forever. In the early days that was fine. But
on a high -speed PCI bus, waiting for that weak
resistor to fill the wire's capacitance would
take several full clock cycles. Which ruins the
speed. Totally. The system would just be sitting
there waiting for the voltage to slowly drift
high enough to be recognized as a valid state,
completely ruining your high -speed operation.
So how did they fix the dripping faucet? The
article mentions Intel created a convention called
sustained tri -state. They essentially changed
the rules of engagement. The sustained tri -state
protocol physically requires that whatever device
is currently talking on the bus must do something
a bit counterintuitive. What's that? It must
forcefully drive the control signals to a high
voltage for at least one full clock cycle before
it's allowed to enter the disconnected high Z
state. Wait, really? So the device basically
has to yell, I'm hanging up, knees no W, by blasting
the signal high before it disconnects? Yes. It
actively pre -fills the wire's capacitance by
using its strong internal transistors to forcefully
drive the bus high. It rapidly forces the wire
to the default state in a fraction of a nanosecond.
And then, and only then, does it drop into high
Z. That's brilliant. So the weak pull -up resistor
isn't responsible for changing the voltage of
the wire anymore. It's only responsible for holding
it there against minor leakage currents after
the chip does the heavy lifting. Exactly. It's
a beautiful workaround for the messy analog physics
of high -speed wiring. And actually, Intel utilizes
this exact same sustained tri -state trick on
the low pin count bus, too. This constant ongoing
battle between pristine digital logic and the
messy reality of physical wiring is just amazing.
It really is. But it also leads us to the final
major revelation in your source material. Because
it turns out tristate logic isn't the only way
to solve this highway traffic jam. No, it's not.
The article mentions a major alternative. the
open collective output. And this fundamentally
changes the rules of the room we've been talking
about. The most famous example of this is the
I2C bus, which is a wildly common bidirectional
communication standard used to connect sensors
and microcontrollers. OK, so how does an open
collector room differ from a tri -stay room?
Well, in an open collector setup, the transmitting
devices physically cannot drive the communication
line to a high voltage. Wait! They can't? No.
They literally lack the internal silicon hardware
to push a positive voltage onto the wire. The
bus completely relies on those pull -up resistors
we just talked about to keep the shared communication
lines held high by default. Okay, let me try
a different analogy for this one. So the I2C
bus is basically a room where the door is held
wide open by a heavy bungee cord, the pull -up
resistor. Okay, I like it. Anyone in the room
can grab the handle and pull the door shut to
signal a zero, but nobody in the room is physically
capable of pushing the door wider open. That
visual works perfectly. To communicate a zero,
you actively pull against the bungee cord. To
communicate a one, you don't push, you just let
go of the handle. You just let go. Yeah, you
let your output float and the bungee cord, the
resistor, snaps the door back open. That's incredibly
elegant because think about the short circuit
problem from earlier. If device A tries to send
a zero, it pulls the door shut. If device B gets
confused and also tries to communicate at the
exact same time, The worst thing it can do is
also grab the handle and pull the door shut or
just let go. It entirely prevents bus contention
by design. No two devices can ever short -circuit
the system by driving high and low simultaneously
because nobody has the physical ability to drive
high. That solves the whole problem. Now in the
old days microcontrollers used to have very rigid
fixed pins. A pin was either a tri -state push
-pull output or an open collector and you had
to buy the right chip for the job. But the sources
say modern microcontrollers are incredibly flexible,
right? They have general purpose input output
pins that can be programmed via software to be
any of those configurations on the fly. Yeah,
which is an absolute dream for modern electronics
design. It offers incredible versatility. But
I do have one final piece of pushback here. Let's
hear it. If tristate logic is so phenomenal for
shared motherboards and open collectors are so
great for sensors, why does the article explicitly
warn that tristate logic is not recommended for
on -ship connections? I mean, why would engineers
completely ditch this elegant system inside the
actual microscopic silicon of a processor and
use something called multiplexers instead? That
warning really comes down to the staggering difference
in scale and the rigorous math required to build
modern chips. Scale and math, okay. connecting
different separate chips on a printed circuit
board interchip communication tri -state is perfect.
But inside the microscopic silicon of a single
processor on chip communication things operate
at blistering Gigahertz speeds. And the physical
rules change at that microscopic level. The rules
of verification change. Fabricating a modern
silicon wafer takes months and costs hundreds
of millions of dollars. If a microscopic signal
arrives at its destination, even a fraction of
a picosecond late, the processor might calculate
2 plus 2 equals 5. Oh, wow. The entire chip is
garbage. So before they ever manufacture the
chip, engineers run a massive software simulation
called static timing analysis. So they mathematically
prove that Every single signal will arrive exactly
when it's supposed to, down to the picosecond.
They have to. But a shared internal bus with
multiple tri -state drivers means the physical
path the signal takes changes dynamically depending
on who is talking and who is in high Z. Right,
it's constantly shifting. It introduces an it
depends variable into the math and static timing
analysis algorithms absolutely despise it depends.
It makes mathematical verification incredibly
difficult if not computationally impossible at
modern processor speeds. Ah. So dynamic paths
ruin the math. So instead of having a shared
wire where chips take turns turning on and off
They use multiplexers. Right. A multiplexer is
basically a highly rigid functional switchboard.
You hardwire every single internal device's output
into the multiplexer on its own dedicated wire,
and the multiplexer acts as the sole central
gatekeeper, physically selecting which input
gets to proceed to the next stage. So it's much
more structured. Yes. It requires vastly more
microscopic wiring, but mathematically the timing
is 100 % predictable. It proves that in engineering,
the environment always dictates the solution.
It really does. I mean, what works on a six -inch
motherboard will completely ruin a six -millimeter
microchip. Exactly. So let's look back at the
journey we've just taken through your sources
today. We started by dismantling the simplistic
myth of purely binary ones and zeros. We discovered
the high Z state, this vital ghostly third state
of total physical disconnection that acts as
the ultimate traffic controller for our hardware.
We saw how memory chips perform that delicate
latency -saving dance between output enable and
chip select, balancing power consumption against
wake -up times. And we battled the messy analog
physical reality of floating wires and unintentional
batteries using weak spring resistors and Intel's
sustained tri -state trick. Right. And finally,
we saw how the rules completely rewrite themselves
again with the bungee -cord logic of open collectors
and the predictable math of multiplexers. It
is just an absolutely staggering amount of unseen
And it is all happening completely invisibly
to the user. That is the wildest part for me.
I really want you, the listener, to think about
this the next time you sit down at your laptop
or pick up your phone. Every single time you
press a glass screen, every time you type a letter
on a keyboard or save a photo, this wildly elaborate
microscopic choreography of devices waking up,
forcefully taking control and gracefully disconnecting
into the void is happening billions of times
a second inside your device. All perfectly synchronized,
all just to keep the wires from burning up. And
if we step back, I think there is a profound
broader takeaway here beyond just the silicon
and copper. What's that? closely, our most advanced
high -speed technology relies absolutely on a
dedicated state of disconnection. That's true.
The entire system only functions because individual
components have the baked inability to actively
step back, go silent, and let others speak solely
to prevent total system chaos. I love that. It
really makes you wonder... If the most complex
processors in the world require a dedicated state
of disconnection just to survive, maybe there's
a lesson there for our always connected, always
shouting human networks, too. Thanks for joining
us on this deep dive. We'll see you next time.