Using a cheap oscilloscope to decode serial communication

Serial interface is one of the oldest and simplest communication methods used in computing. Back in the day it was used to connect a terminal to a mainframe, and it's still a great way of connecting computers that have no common way of talking to each other, like Commodore 64 and PC or TRS-80 and Amiga. Nowadays serial interfaces are widely used to communicate with microcontrollers. If you ever played around with Arduino or alike, and tried to do more advanced projects than blinking LEDs, you probably talked to a microcontroller via some sort of serial connection.

But no matter if you just used some wires, a null modem cable, or an USB, you probably ran into a situation where the communication didn't work. It's hard to find the source of the problem if you don't even know if it's in hardware or in software. The best way to determine if the hardware part is working correctly is to use an oscilloscope with a built-in serial decoder. But such oscilloscopes are very expensive and if you are not a professional electronics technician chances are that you don't own one. Or maybe you only have a basic model, without a serial decoder? Don't worry, it will do. I have been using a simple DSO 150 clone with success, and I paid for it less than 30 dollars.

Software setup

To demonstrate how to use a digital oscilloscope to decode serial communication we will use a very basic setup, consisting of an Arduino Uno, a couple of wires, and a simple program. Let's start with the program first:

void setup() {
  Serial.begin(9600);
}

void loop() {
  Serial.print("a");
  delay(1000);
}

Use Arduino IDE to compile and upload the program. Now you can open a serial monitor in the tools menu to verify if it works. You should see a letter "a" printed out to the screen once a second - it means that the transmission is working correctly.

Hardware setup

However, your computer is not connected to the serial line of the Atmega328 microcontroller directly. The signals are being translated by a separate chip and sent through USB. But you can still hook up to the serial interface itself, using only three wires. Look for pins RX, TX and GND on the Arduino board. RX stands for "receive" and TX for "transmit". The voltage level on those pins depends on the board, but in case of Arduino Uno it's 5V. Because you have current flowing through those pins, you also need a ground pin (GND). If you have a serial to usb converter, you can connect the Arduino to the converter using the following wiring:

RX  <---> TX
TX  <---> RX
GND <---> GND

It's the simplest serial cable you can build. There are other pins as well, but they are not necessary for the connection to work. Now you can use a terminal program connected to the serial port used by the converter to watch the transmission. When you power up the Arduino (through USB or PSU) you should see the same result as you've seen with the serial monitor: letter "a" printed out once a second.

Connecting oscilloscope

To debug a serial connection you need to use an oscilloscope. Connect a probe to the TX pin, and a ground to the GND pin:

Now set the vertical resolution to 2V and the horizontal one to 0.2ms. Switch the oscilloscope to single mode, so that it holds a waveform when it detects data on the transmission pin. After approximately one second, you should see the following picture:

Decoding serial data

After you managed to capture the data, it's time to decode it. As you can see, the default serial line state is high (logical 1), and a single pulse (which represents one bit) takes around one half of a square field on the oscilloscope screen. Standard parameters for the serial transmission are 8N1, which means that one data packet consists of 10 bits: a start bit (which is always 0), 8 data bits (from lowest to highest), and a stop bit (which is always 1). The bit pattern shown by the oscilloscope should now be clear:

When you remove the start and the stop bits, and rearrange the data bits from highest to lowest, you are left with binary number 01100001, which is decimal 97. Now take a look at the ASCII table and search for code 97. It stands for the letter "a".

Summary

Debugging serial communication can be a tedious task, although sometimes necessary. But, as it turns out, it can be done with very simple tools, without an expensive serial decoder.