How reliable is the USB serial port connection

I was wondering how reliable the USB serial port connection is between the Arduino (I am currently testing with old Diecimalia unit) and PC.

I noticed that there was something funny going on with the communication, and found that the bytes I was sending were being either dropped or corrupted.( I made sure that recieve buffers were not overflowing)

I assumed that the underlying hardware would take care of the error handling, but I guess this is not a good assumption? What is the easiest way to ensure through software that I have more stable communication?

I assumed that the underlying hardware would take care of the error handling, but I guess this is not a good assumption?

No. Serial communications does not include error detection nor error correction. That is your responsibility.

USB, however, does provide error detection and retry.

What is the easiest way to ensure through software that I have more stable communication?

In my experience, the best place to start is to determine why the errors are occurring and try to eliminate the source of the problem. The second step is to introduce a way to detect errors; a CRC included with the data works very well.

This would be a good time to offer more details. The "serial connection" on an Arduino is extremely short which lends itself well to reliable communications. It is unlikely to have communications problems between an Arduino and a PC.

I guess I'm just connecting my PC via a USB cable (normal length, less than 1 ft) to the Arduino, with a baud rate of 256000. But I also get same issue with a slower baud rate. I am just using the Hardware Serial library on Arduino and on the PC side I am just using Window's sockets API to access the com ports.

You say that USB does provide error detection and retry? Is this just handled transparently?

Window's sockets API to access the com ports.

You can't be. The sockets API does not work with serial ports.

You say that USB does provide error detection and retry?

Yes.

Is this just handled transparently?

Yes.

On the Arduino, do you have anything connected to pin 0 or pin 1?

I guess sockets is not the right term. This is what I tried to use
Serial Communications in Win32 | Microsoft Learn and I guess its standard way of doing it.

nothing connected to pin 1,

If USB transparently handles error detection and retry, there should be no need for me to implement this myself in arduino software right?

I am currently testing with old Diecimalia unit

with a baud rate of 256000.

I'm pretty sure that the maximum supported baud rate is 115200.

The issue with baud rates is that they do not exactly match 16MHz (or 8MHz) frequencies, so, some adjustments are made. Usually, these work OK. If your crystal or voltage to the crystal is off a little, and the PC is ticking a tiny bit fast, some errors can occur.

I guess sockets is not the right term. This is what I tried to use
Serial Communications in Win32 | Microsoft Learn and I guess its standard way of doing it.

Are you using that sample as-is or have you made changes to it?

nothing connected to pin 1,

What do you have connected to pin 0?

If USB transparently handles error detection and retry, there should be no need for me to implement this myself in arduino software right?

In most case the answer is "yes". In an environment with a great deal of electrical noise, I assume it's possible for that noise to cause communications problems.

If USB transparently handles error detection and retry, there should be no need for me to implement this myself in arduino software right?

The connection between the PC and the USB chip should be error-corrected. The connection between the USB chip and the Arduino is NOT, and is only “barely” capable of speeds faster than 115200. (Try v0022; the serial code has been optimized a bit…) (and the closest actual bitrate supported by a 16MHz AVR is 250kbps; 2.3% away…)