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2 July 2023

Serial Experiments Anne - Part 1

by Anne Macedo

IDK who made this but I love it

I love Serial IO and UART ports. I mean, they are so basic: send bits at a specific rate, connect tx and rx, read bits at a specific rate and then BAM, you have a TTY, you have logs.

However, if timing (or voltage) is not precise, things take a turn to the worse.

I’m currently thinking about a project where I use the COM port of my motherboard to send the serial logs to an FPGA, which receives these logs and renders them on a display. I kind of like doing things headless while I’m on my Mac and I usually prefer to do things via SSH, but I also want to have the feedback of the serial port.

In order to test things before the FPGA arrives, I wanted to use a 5V USB to TTL to read the logs from my 12V COM port. Nothing fried lol.

COM Port Pinout

It worked, but not consistently… and this is so annoying.

Mambo mambo jambo!

BTW, I needed to add a serial console to the kernel command line. More here [2].


This will allow you to have a serial TTY on /dev/ttyS0 at a baud rate 115200.

Analyzing stuff with a Logic Analyzer

Checking some signals in the logic analyzer, I saw the inverse measurement is 50khz for some bits. Some others are showing 25khz.


Things were looking wrong… I’ve increased the capture to 24MS/s and the bits from the 115200 baud rate were showing… but still had framing errors.

So! I decided to invert the signal and AAAAAAASUS!


It was inverted all along… how do I invert this signal to my USB to TTL? It was probably working sometimes because of USB-C or something.

Seems like my CP2102 doesn’t support inverting the signal, so I’m buying an FTDI.

I bought an FTDI… it didn’t change anything. I could actually invert the bits on the FTDI, but for some reason I couldn’t reprogram it with FT_Prog.

What actually happens is this:

TTL to RS232 voltage

The COM port on my motherboard runs on RS232 – where it idles in +12V and every bit is -12V. TTL, on the other hand, idles on 0V and every bit is 5V. That’s why the signal was inverted!

After spending a long time trying to understand it, and discussing with folks from the Hardware Hacking group, I decided to buy a MAX232.

MAX232 Pinout

This chip translates RS232 to TTL logic, inverting the signal!

RS232 to TTL conversion

After beating my head a lot, things got quite easy. Datasheet is on [1].

Basically, if I want to translate RS232 to TTL, this is the logic:

COM Port Tx (RS232) -> MAX232 R2IN pin -> MA232 R2OUT pin -> FTDI Rx

And the TTL Rx to RS232 - which I didn’t bother so much:

FTDI Tx -> MAX232 T2IN -> MAX232 T2OUT pin -> COM Port Rx (RS232)

The COM Rx pin is the NSIN (pin 2) and COM Tx pin is the NSOUT (pin 3).

I plugged it all and the logic was still wrong… what am I missing?

Okay! So, it idles at 5V… where does it get this voltage? Air? No! VCC!

So I plugged the FTDI VCC to the MAX232 VCC and VOILÁ! Serial Logs!


Next steps

As I mentioned, now I have the circuit for translating RS232 to TTL. Next time, I’ll need to find out how to program the FPGA to:

  1. Be able to receive signal from UART
  2. Turn this signal into pixels
  3. Send these pixels to the LCD

But we’ll talk about it next time :).


[1] MAX232 Datasheet

[2] Linux Serial Console

tags: hardware - serial - uart