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Topic:   Odd Baud Rates

By: ScottPosted on: Feb 7 2017 at 02:26:19 PM
In my line of work, I often run into equipment that needs odd Baud rates.

Currently, I need 9660 for a MARKIII controller made back in 1986.
I know that the hardware of the PC can accept that rate, because they have a test program that does it. But to date, I can only find libraries that limit a programmer to the norms. 1200,2400,4800,9600...etc.

My first question is, does your product allow control over the UART, or do you also limit the control to just norms.

Second, why in the world would do you think a programmer would think it was a good idea to limit the baud rates like this?

By: GuestPosted on: Feb 7 2017 at 10:55:33 PM
The baud rate of a UART is controlled by a Crystal. For example a 1.8432 MHz attached to the UART.

The uart achieves baud rates by dividing that frequency into a bit rate. So, crystal divided by 16 = 115200

Crystal divided by 192 = 9600

A baud rate of 9660 is not possible using a standard 1.8432MHz crystal.

Having said that. Modern USB adapter, virtual com ports etc that don't have a UART in the true sense of the word may well be able to do strange baud rates but it would be up to that device's driver to decide if it wanted to allow non-standard baud rates.

It wouldn't surprise me if 9600 was so close to 9660 that a PC running at 9600 might well be able to communicate with your markiii controller. Have you tried?

Your second question - why would anyone limit baud rates to common baud rates. I guess it's a case of meeting the requirements of most users most of the time. Allowing a user to choose a baud rate of 12345 bps (just a made up number probably not supported by any hardware) would have that user demanding tech support when his application didn't work.

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