The relationship between Frequency and bps

janetsmith2000@yahoo.com · Jul 5, 2005

Hi, I am connecting a mobile equipment to RS232.

On the board of mobile equipment has a 3.579Mhz Oscillator.

when I run the software (which come along with mobile equipment), I choose [9600 bps 3.57MHZ] as the speed to communicate with the board.

The full list of speed I can choose are
9600 bps 3.57MHz
10472 bps 3.89 MHz
11520 bps 4.28 MHz
12800 bps 4.76 MHz
14400 bps 5.35 MHz
16457 bps 6.12 MHz
19200 bps 7.14MHz

But only [9600 bps 3.57MHz] works. The rest cann't communicate with the board.

My question is:
What is the relationship between bps and Hertz?

instruite · Jul 5, 2005

bps is bits per second and normally one bit is send in one cycle so
bps = Hz

The reason for

But only [9600 bps 3.57MHz] works. The rest cann't communicate with the board.

is

On the board of mobile equipment has a 3.579Mhz Oscillator.

to work with other settings you will have to change the crystal on the board

janetsmith2000@yahoo.com · Jul 6, 2005

if bps = Hz,
then why [9600 bps 3.57Mhz]? seems like 9600 bps = 3.57Mhz.

instruite · Jul 6, 2005

well it doesnot say
9600bps = 3.75MHz

The full list of speed I can choose are
9600 bps 3.57MHz
10472 bps 3.89 MHz
11520 bps 4.28 MHz
12800 bps 4.76 MHz
14400 bps 5.35 MHz
16457 bps 6.12 MHz
19200 bps 7.14MHz

This shows the settings for the communication
9600 bps 3.75Mhz
means when the mobile board is having a crystal oscillator of 3.75MHz
The settings in the processor used on mobile will be for 9600 bps

The serial communication is not the only task for a processor
The processor used on mobile board will be doing various other tasks like (for example) receiving reception from the network at the same time as you are carrying out the communication on RS232

So there are normally certains settings in the processor to carry out the serial communication at a certain bps depending on the input crsytal frequency

In your case the settings are constant so bps is function of input crystal frequency

So to have higher bps you need to have higher crysal frequency

mstechca · Jul 7, 2005

have you thought that extra information could be included in the 9600bps figure, including start and stop bits?

Make sure your communications equiptment is capable of exceeding 9600bps.

gerty · Jul 7, 2005

Something often overlooked.. Make sure both devices (transmitter/reciever) are set for the same baud rate...

_3iMaJ · Jul 8, 2005

The baud rate and clock frequency are two very different things. You have to look into what kind of modulation scheme the RS232 interface uses to figure out the relationship between the clock and baud rate.

If you can tell me the type of modulation used then I can probably find an expression to relate the clock to baud rate.

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The relationship between Frequency and bps

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