Serial baud rate

[Graham McKechnie]

Hi,

I've found while testing baud rates on a couple of GPS units, that the
baudrate does not seem to be critical on my BT unit, but it is on my compact
flash unit

I only found this by accident on the BT unit. Normally when I set the baud
rate for the BT unit I just use what the manufacturer recommended ie 57600
baud. However since I was looping through the bauds lowest to highest
4800-57600, the code tested 4800 first and found a GPS unit and consequently
exited my loop. I then actually tested the BT unit at 4800 and it works just
fine. I haven't tested this, but I presume it will work at any baud rate up
to 57600. The compact flash unit will only work at the rate recommended by
the manufacturer ie 4800.

Some questions

As I've only got one BT unit an Emtac CruxII/BTGPS, is this common behaviour
of BT units?
Could anyone with a BT unit - post its baud rate and manufacturer's name?
Is there any disadvantage running them at slower baud rates? Do they
actually run slower?
Are there any BT units that work at a higher bauds than 57600? Or is there
likely to be in the future?
What is different about BT being more flexible as compared to the older
style compact flash units?

Graham

 

[Ilya Tumanov [MS]]

My guess would be simulated serial port over BT does not actually set
particular speed.

After all, there's probably no actual UART involved, data is transmitted at
whatever rate BT allows for.


Best regards,

Ilya

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[Guest]

Hi,

I have a gps BT device by Holux. Recommended baud rate is 38400. Setting it
to minimum 4800 works fine. I guess lower baud rate affects only the
tramission rate.

 

[DickGrier]

Hi,

There is no need to use a speed higher than 4800 bps (usually) because that
is, by far, the most common speed that the NMEA-0183 data stream from the
actual receiver generates is 4800 bps.

In the BT unit, this data is tranfered via RF, and the modulation is only
the data, but not the raw speed of the underlying serial data. So, while it
may work at higher "virtual serial port" speeds, there is nothing to be
gained by using these higher speeds.

I have seen a few GPS receivers (non-Bluetooth) that used higher serial
speeds -- in fact, I designed one back in 1993 that output NMEA-0183 at 9600
bps. I had some real-time considerations where I needed to deliver a full
NMEA stream in less than 0.5 seconds, thus I used the higher speed. This is
not a common requirement.

Dick

--
Richard Grier, MVP
Hard & Software
Author of Visual Basic Programmer's Guide to Serial Communications, Fourth
Edition, ISBN 1-890422-28-2, Mabry Publishing (391 pages, includes CD-ROM).
July 2004.
See www.hardandsoftware.net for details and contact information.

 

[Graham McKechnie]

Thanks guys for your comments.

Graham