Newbie Bluetooth Question re: default comm data rates - Page 1
Forum section: electronics / beginners

author: Dundarave
author_rank: Regular Contributor
timestamp: February 24, 2019, 07:02:30 am
content: Since I can't find an answer despite a robust Google effort, I'm sure this must be an incredibly stupid question...I'm playing with three Bluetooth modules, one an HC-05, another embedded in a Soma Smart Shades blind motor, and a 3rd embedded RPi Bluetooth module, trying to figure out how to get the HC-05 (connected to an Arduino Nano) to talk directly to the blind motor without needing to use the HCI & HomeBridge code via the RPi. I.e. use the HC-05/Arduino combo to simply emulate the 4 or 5 commands I need to send to the Soma blinds device without needing to use the whole RPi/HomeBridge enchilada.My question is this: how does a Bluetooth master know at which data rate to initially talk to an arbitrary slave device (i.e. one it has not connected with before)? Default commands for HC-05 are at the 38400 rate, and default on-line data rate is 9600, but they can both be changed manually. The HCITools program on the RPi can return various inquiry and scan data, but I see nothing returned from any device about its presently-configured (i.e. on-line) data rate.I ask because while the RPi can "inquire" and get data from both the HC-05 and the Soma blind transceiver, I cannot get the Soma transceiver to respond to the HC-05 module via a connect/scan/inquiry command, and I'm thinking that until I confirm that the current Soma blind transceiver on-line data rate is 9600 (or not), I'm just wasting my time.For example, if the Soma blind device has been set for on-line data at 19200, how would the HC-05 ever find out? The RPi seems to be able to figure it out, so I'm thinking that there must be a well-known convention that deals with this, but I sure can't find it. Does it perhaps simply run through all the data rates until it gets a valid response? If so, I would have hoped that it would display any data rate-finding result in the inquiry response...Thanks for any insight...

author: Dundarave
author_rank: Regular Contributor
timestamp: February 24, 2019, 06:40:30 pm
content: Quote from: Dundarave on February 24, 2019, 07:02:30 amFurther googling has sent me down an interesting rabbit-hole of low-energy Bluetooth vs legacy types, and the Linux BlueZ Bluetooth stack with its associated command line tool set. I can so far see that the answer to my question is ultimately buried in the sophisticated Bluetooth master-slave handshake protocols, and that these appear to be done on a "well-known" Bluetooth channel at some standard data rate.I'll eventually follow up with a summary of what I find re: connecting (and controlling) the standalone HC-05 Bluetooth module to the Soma window blind motor for those interested. I love getting "down to the metal" with new (to me) technologies.

author: mvs
author_rank: Frequent Contributor
timestamp: February 24, 2019, 06:57:31 pm
content: Quote from: Dundarave on February 24, 2019, 07:02:30 amBluetooth SPP profile is just emulation of UART, there is no fixed/defined bit rate like in UART. If you send a symbol between RPi and HC-05, it will be transmited at available bluetooth link speed, which can be some hunderts of kbit/s.

author: Karel
author_rank: Super Contributor
timestamp: February 24, 2019, 07:37:58 pm
content: In practice, using Serial Port Profile you can go up to approx. 40 KBytes/sec.Keep in mind that there's no minimum speed, at least because you have to share the 2.4GHz band with other devices(Bluetooth, Wi-Fi, Industrial, Scientific, Medical, Microwaves, etc.).https://www.linux.org/threads/bluetooth-and-linux.4555/https://en.wikipedia.org/wiki/ISM_bandIn short, there's no manufacturer of Bluetooth modules that will guarantee that you can transmit at least one byte per second over a distance of 1 meter...