[ale] setting non-standard baud rate on serial port

[Byron Jeff]

On Tue, Apr 26, 2011 at 05:31:47PM -0400, Steven A. DuChene wrote:
> I have an automotive diagnostic device that seems to only want to communicate
> across the serial port at a baud rate of 8192 which is a non-standard baud rate.
> When I try to use setserial or stty to set this baud rate on the port both
> programs tell me "Invalid Argument"
> 
> stty -F /dev/ttyS0 8192
> stty: invalid argument `8192'
> Try `stty --help' for more information.
> 
> Anyone have any idea how to set baud rate on a serial port to a non-standard baud rate like this?

It took a minute for me to remember how to do this. The command you need is
setserial. It configures your serial port from the command line.

The trick setserial used was to replace the standard 38400 BPS rate with a
one of a set of nonstandard bit rates. These include:

	  spd_hi        use 56kb instead of 38.4kb
          spd_vhi       use 115kb instead of 38.4kb
          spd_shi       use 230kb instead of 38.4kb
          spd_warp      use 460kb instead of 38.4kb
          spd_cust      use the custom divisor to set the speed at 38.4kb
                                (baud rate = baud_base / custom_divisor)
          spd_normal    use 38.4kb when a baud rate of 38.4kb is selected

It's the spd_cust that you are interested in. It will take the base speed
of the UART and divide it by a custom divider. I just ran setserial on a
local serial port and got this:

$ setserial -a /dev/ttyS0
/dev/ttyS0, Line 0, UART: 16550A, Port: 0x03f8, IRQ: 4
Baud_base: 115200, close_delay: 50, divisor: 0
closing_wait: 3000
Flags: spd_normal skip_test

So the Baud_base is 115200. Quick calcuation:

$ bc
bc 1.06.95
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 Free Software Foundation,
Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'. 
scale=5
115200/8192
14.06250

setserial has a divisor command to set the divisor. So if the divisor is
set to 14 the speed will be 115200/14 = 8228 BPS. Another quick
calculation:

8228/8192
1.00439

Any bit rate that's within 2 percent of nominal generally will work. With
the error here being less than 0.5 percent, it should work like a champ.

Hope this helps. It's been 10 years since I've looked at setserial. I used
it to talk to one of my modems at 230 Kb/s

BAJ



> --
> Steven DuChene
> 
> 
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-- 
Byron A. Jeff
Department Chair: IT/CS/CNET
College of Information and Mathematical Sciences
Clayton State University
http://cims.clayton.edu/bjeff