Why do screen and minicom drop incoming bytes from a serial port while cat does not?

Kayson

3/1/23, 7:27 PM

I recently needed to reset the password on an old APC UPS which requires connecting their proprietary serial cable, hitting the reset button, and logging in with the default username and password over the serial interface. I connected it to the serial port on an R720XD, and passed the port through to a Ubuntu VM.

I used stty to configure the serial port according to APC's documentation, launched screen /dev/ttyS0 <options>, and went to work. For some reason, the output was a total mess. It seemed like it would pick up the first 3-4 characters of every transmission, then miss the rest. I tried minicom with the same results.

So I did a loopback test - connected RX and TX together - to see if what I sent would come back in. Same issue. I hooked up an oscilloscope to the wire and discovered that every keypress was sent properly; I could see it going over the wire. But for some reason, most of the characters wouldn't show up on screen.

I then discovered that if I did cat /dev/ttyS0 in one terminal, and echo alongstring > /dev/ttyS0 in another, I could see everything just fine!

I ended up just leaving the cat open to view the incoming bytes, and running screen in another terminal to send keypresses, which worked just fine and was a lot easier than using echo over and over. Password was successfully reset using this.

But now I'm curious... does anyone have any ideas why screen and minicom had issues? I tried every option I could find in man, but no luck. It seemed like there might be something else pulling the bytes out of the buffer, but I have no idea what that could be.

0 + 0

linux

serial

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: Why do screen and minicom drop incoming bytes from a serial port while cat does not?

TH: เหตุใดหน้าจอและมินิคอมจึงทิ้งไบต์ขาเข้าจากพอร์ตอนุกรมในขณะที่ cat ไม่ทำเช่นนั้น

RO: De ce ecranul și minicom elimină octeții de intrare de la un port serial, în timp ce cat nu?

RU: Почему screen и minicom отбрасывают входящие байты из последовательного порта, а cat нет?

VI: Tại sao màn hình và minicom thả các byte đến từ một cổng nối tiếp trong khi mèo thì không?

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