What does serial data look like on an oscilloscope?

Question

How I can find the data field on oscilloscope at 300 baud or at 9600 baud? Could someone show me picture on oscilloscope plase.

This is the picture Oscilloscope image

at 9600 Baud

• James Waldby - jwpat7: At google images, type scope rs232 into the search box.

Answer 1

I had these images lying around so here is an example of "Fab" being sent at 9600 baud:

RS232 - 3 letters

And now a single byte (the letter "F"):

RS232 - one byte

• A - no data (Tx is high)

• B - The "start bit". The line is taken low to tell the receiver that a character (byte) is commencing to be sent. The receiver waits for one and a half clock times before sampling the line.

• C - First character arrives (the letter "F" or 0x46 or 0b01000110). There is no clock bit as such, the incoming data is simply sampled at the baud (transmission) rate. In contrast to SPI communication the data arrives least-significant bit first (in case you are not sending 8 bits per byte). Thus we see 01100010 (rather than 01000110).

• D - The stop bit. This is always high, to ensure that we can distinguish between the end of this byte, and the start of the next one. Since the start bit is a zero, and the stop bit is a one, there is always a clear transition from one byte to the next.

• E - The start bit for the next character.

---

The time for one bit is shown on the logic analyzer as 0.1041667 ms (that is, 0.0001041667 seconds). This agrees with the inverse of the baud rate:

1 / 9600 bits per second = 0.0001041667 seconds per bit

---

Looking at your photo, my guess is you have inverted logic here. That is, RS232 levels (where 5V is a 0-bit and 0V is a 1-bit). Thus (and this is a guess because all the bytes are close together):

One RS232 byte

I marked the start and stop bits, and put a dot for each bit inbetween.

---

Also see What is Serial.begin(9600)?

• Gerben: Note that there are quite a few different setting on serial, besides just the speed. Parity bits, Stop bits. Data size.