Decoding Serial Data with an Oscilloscope

OSCILLOSCOPE

V/DIV2.00V TIME/DIV104µs

Three keys to reading the signal

What to watch for on the scope

01

Find the falling edge

The wire idles HIGH. The moment it drops to LOW, a new character has just begun. That LOW pulse is the start bit - it wakes the receiver and starts the clock.
02

Count ten bit slots

Every character is one tidy group of 10-bits: one LOW start bit, eight data bits carrying the letter, and one HIGH stop bit to finish. Same width every time. Count them off as the trace scrolls.
03

Each slot takes 104 microseconds

Both devices agreed on speed before talking - this trace runs at 9600 baud, so each bit slot is about 104 µs wide. That is why the slots line up perfectly with the table below.

Three keys to decoding the signal

How to turn a trace into a letter

01

Smallest place value comes first

This is the part that surprises everyone. On paper we write the biggest digit on the left (128, 64, 32...). UART sends the opposite way: the smallest place value (1) comes first, right after the start bit. Then 2, 4, 8, 16, 32, 64, 128. Read left to right, but the values grow as you go.
02

HIGH means add that value

Look at each bit slot. If the trace is HIGH, add that slot's place value to your running total. If it is LOW, add nothing. Skip the start bit and the stop bit - they are the structure, not data.
03

Look up the total in ASCII

Your eight data bits add up to a number between 0 and 255. That number is an ASCII code. Find it in the table and you have recovered the letter.

Digital basics

Digital only knows on and off

Voltage for a digital signal is either near 5 V or near 0 V. Nothing in between counts. Engineers call these two states HIGH and LOW, and they stand for the binary digits 1 and 0.

Every message, photo, song, and video your devices exchange is built from long streams of these two states. That is the entire alphabet of digital electronics.

Why the format?

Receivers are listening

The receiver does not know when a new character will arrive; the signal could sit idle for milliseconds or hours. So UART wraps each byte in a frame with a built-in signal: a LOW start bit.

That falling edge is the beginning. It says "a character begins right now," and the receiver clocks out the next ten bit slots automatically. The HIGH stop bit at the end says "I'm done, line is idle again."

Letters ride as numbers

ASCII is just a lookup table

The wire cannot carry the letter 'A' directly. It can only carry HIGHs and LOWs. So the sender looks up 'A' in a table called ASCII and finds the number 65. In binary, that is 01000001.

Those eight bits travel down the wire. The receiver looks up 65 in the same table and gets the letter back. You will use this lookup yourself in Decode mode.

Why smallest-first?

It is a UART convention

You read numbers on paper biggest-first. UART transmits smallest-first. There is no math reason for this - it is simply the convention the protocol was built around, and every UART chip in the world follows it.

So the rule is just: trust the bit value labels. The 1s place is always the first bit after the start bit.

Reading the trace

Bits line up with the table

The trace shows voltage changing over time. Time runs left to right, and every bit occupies the same width - one clean bit slot. The table below the scope lines up with those slots, so you can read directly without measuring.

Look at each slot and decide: is the line HIGH or LOW right now?

When in doubt

Check the stop bit

If your decoded number lands on something strange or the last bit is not HIGH - you probably miscounted where the data began.

Find the next falling edge, treat it as the start bit, and count ten slots forward. A clean data always ends with the stop bit HIGH.

Message to transmit

Decimal

Binary

Enter a character above (or click a preset) then press NEXT BIT to begin walking through the 10 bits that define it.

Press NEW PUZZLE to start.

Current character

ASCII reference

Glossary of terms

Serial communication: Sending data one bit at a time down a single wire. The opposite of parallel, which uses many wires side-by-side. Serial uses fewer wires, so it's how almost everything talks to everything else: USB, Ethernet, HDMI, and the link between a microcontroller and your laptop are all serial under the hood.
Bit: The smallest piece of information a computer handles: a single 0 or 1. On a wire, a bit is one short stretch of voltage - either HIGH or LOW.
Byte: A group of 8 bits. One byte can represent 256 different values (0 through 255), which is enough to label every letter, digit, and symbol you see on a keyboard. UART sends one byte in a group.
Binary: A number system that uses only two symbols, 0 and 1. Every signal in a digital circuit is one or the other, so binary is the natural language of computers.
Oscilloscope: An instrument that draws a graph of voltage versus time. The horizontal axis is time; the vertical axis is voltage. Engineers use it to see what an electrical signal is actually doing - not just whether it's on, but exactly when it switches and how cleanly.
Trace: The line the oscilloscope draws on its screen. Each point on the trace is the voltage on the wire at one moment in time. Reading a signal means looking at where the trace is HIGH, where it's LOW, and exactly when it switches.
HIGH: A voltage level near the supply voltage, typically 5 V or 3.3 V. In standard logic this represents a 1.
LOW: A voltage level near 0 V. In standard logic this represents a 0.
Idle: The resting state of a UART wire when nothing is being sent: held HIGH. Idle gives the receiver a stable starting point, so the first drop to LOW is unmistakable.
UART: Universal Asynchronous Receiver-Transmitter. The hardware inside a microcontroller that performs serial communication. Asynchronous means the two ends do not share a clock; they just agree on the bit rate ahead of time.
Baud: The number of signal changes per second on a wire. For UART, one baud equals one bit per second. 9600 baud means each bit lasts about 104 microseconds.
Microsecond (µs): One millionth of a second. At 9600 baud each bit lasts roughly 104 µs - fast for a person, slow for a computer.
Frame: One complete UART transmission of a single byte: a start bit, then 8 data bits, then a stop bit - 10 bits total. The trace shows one frame per character you send.
Start bit: A single LOW bit that announces the beginning of a UART frame. The line idles HIGH; pulling it LOW gives the receiver a clear edge to synchronize on.
Stop bit: A single HIGH bit at the end of a UART frame. It returns the line to idle so the next start bit can be detected.
LSB: Least Significant Bit. The bit worth the smallest amount: place value 1. UART transmits this bit first, which is the opposite of how we write numbers on paper.
MSB: Most Significant Bit. The bit worth the largest amount in a byte: place value 128. UART transmits this bit last.
ASCII: American Standard Code for Information Interchange. A table that maps each letter, digit, and symbol to a number from 0 to 127. 'A' is 65, 'a' is 97, '0' is 48. ASCII is how the sender decides which byte to put on the wire.