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If you search around the internet, you can easily find websites, google images, as well as many (YouTube) videos that explain the various properties of COM/serial/RS232 ports. As far as i'm concerned in most of these they state that in the COM port dialogue box the baud rate can be seen (and not just in Windows OS), such as here, here and even on Sparkfun here. And this is clearly false, since it explicitly states the bit rate. Here's an image from my Windows 8.1 PC as well:

enter image description here

And we know that bit rate isn't the same as baud rate. Also numerous times i've heard people e.g. on youtube videos talking about messing around with the "baud-rate" on windows pc. Now i'm confused. What is going on here. It clearly states the bit rate, isn't that right? Am i missing something?

KeyC0de
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3 Answers3

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Despite being marked "bits per second", that dialog actually displays baud as a rate in symbols per second. (Symbols include data bits but also start, stop, and parity. For serial ports these are often also called "bits".)

Besides framing symbols, the other cause for a difference between bit rate and baud would be multilevel signalling -- however this doesn't apply to PC serial ports since they only use binary signalling, therefore one data symbol = one bit. Don't be confused by the fact that many serial-attached modems use a larger signal constellation, this refers to the link between the modem and computer, not between two modems.

The selections shown in the image in the question will result in 9600 baud, but only 960 bytes per second. (1 byte = 8 bits but due to start and stop intervals, the serial port sends 10 symbols per byte)

Ben Voigt
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  • As per the second paragraph in Ben's answer, there is always exactly one bit per symbol. So bits per second really is the same as baud in this case. – Harry Johnston Feb 27 '16 at 00:08
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    @HarryJohnston: There's exactly one bit per *data symbol*. Framing symbols don't contribute to the transfer rate at all. For the common case of 8 bits, 1 stop, no parity, it averages to 1 symbol = 0.8 (data) bits. – Ben Voigt Feb 27 '16 at 00:13
  • Sure, but a 100Mbs ethernet connection doesn't actually give you 100Mbs of user data either, some of that is ethernet and higher level frames. We still *call* it 100Mbs. I don't see why serial should be any different in that respect. – Harry Johnston Feb 27 '16 at 00:21
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According to this answer: What is the difference between baud rate and bit rate?

It looks like it's due to the fact that with early analog phones, bps = baud rate. ie 1 symbol = 1 bit. That would lead to the assumption that a UI designer at some point simply made some assumptions and mixed the terms based on some expectation that COM ports were going to be used to plug modems in.

Community
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mcr
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Modems don't use a strict digital transmission method, but instead use FSK, which allows for a baud (your "symbol") o be more than one bit (binary data). A phone line has a high frequency limit of about 3300 Hz. If that was the cutoff, your modem couldn't send more than 2400 baud (bit rate). By shifting the signal within one cycle, it's able to transmit more than 1 bit in 1 baud. Add 4 shifts and you up the bit rate from 2400 to 9600.

At least that's what I remember from some 20 years ago.

Joe Voytovich
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