How to get data from body scale as CBPeripheral using BLE (Bluetooth Low Energy) on iOS

Question

I just started using Bluetooth and want to read the body composition measurement from a body scale (Model: adeVital Analysis BA 1401) via Bluetooth.

When set up my iPhone as Central and connect it to the scale (which is CBPeripheral), I can read the scales device information such as Hardware Revision Number, Manufacturer, etc. I can't get the actual measurement data though.

I iterated through all services and characteristics and set the notification flag.

[peripheral setNotifyValue:YES forCharacteristic:aCharacteristic]

for each characteristic. And the delegate method

- (void) peripheral:(CBPeripheral *)peripheral didUpdateNotificationStateForCharacteristic:(CBCharacteristic *)characteristic error:(NSError *)error {

gets called without error. When I check the updated peripheral, I find this:

<CBCharacteristic: 0x1740949b0, UUID = 8A82, properties = 0x20, value = <a1014711 f3000000 00000000 00000000 00000000>, notifying = YES>

This all happens when the scale is turned on (without having measured anything yet). Now when I step on the scale and the measurement is done, it shows a Bluetooth icon indicating a data transfer, the iOS Application is not receiving any notifications though. What am I missing?

So summarized: The peripheral connects to the central and updates a characteristic before it measures, but after that, no more notifications come up.

Could the value in the updated CBCharacteristic be a UUID for a Service to which I would have to subscribe somehow to get the actual data?

Hope someone can help me out here

EDIT:

As it might be relevant, here are the services and characteristics I receive:

Services:
"<CBService: 0x17407dc40, isPrimary = YES, UUID = Device Information>",
"<CBService: 0x174070f80, isPrimary = YES, UUID = 7802>"

Characteristics
"<CBCharacteristic: 0x1740959a0, UUID = Serial Number String, properties = 0x2, value = (null), notifying = NO>",
"<CBCharacteristic: 0x174095900, UUID = Hardware Revision String, properties = 0x2, value = (null), notifying = NO>",
"<CBCharacteristic: 0x1740952c0, UUID = Firmware Revision String, properties = 0x2, value = (null), notifying = NO>",
"<CBCharacteristic: 0x1740958b0, UUID = Manufacturer Name String, properties = 0x2, value = (null), notifying = NO>",
"<CBCharacteristic: 0x174095860, UUID = Software Revision String, properties = 0x2, value = (null), notifying = NO>"

"<CBCharacteristic: 0x170095ea0, UUID = 8A21, properties = 0x20, value = (null), notifying = NO>",
"<CBCharacteristic: 0x170095e50, UUID = 8A22, properties = 0x20, value = (null), notifying = NO>",
"<CBCharacteristic: 0x1700952c0, UUID = 8A20, properties = 0x2, value = (null), notifying = NO>",
"<CBCharacteristic: 0x170095e00, UUID = 8A81, properties = 0x8, value = (null), notifying = NO>",
"<CBCharacteristic: 0x170095db0, UUID = 8A82, properties = 0x20, value = (null), notifying = NO>"

EDIT 2:

The CBCharacteristics have the following properties:

8A20 = Read
8A21 = Indicate
8A22 = Indicate
8A81 = Write
8A82 = Indicate

all other properties are BOOL NO

When I turn on notifications for 8A21, 8A22 and 8A82, I receive an NSData from 8A82 right at the start, when the scale is turned on (so it can’t be a measurement at that point). I assume, the actual measurement data is being updated by the 8A21 characteristic. Yet it won’t notify my delegate and I can’t figure out why.

Viewing the syslog, you can see that the official app of the manufacturer spills the log as follows:

Feb 20 14:19:49 i6S lifesensehealth1_1[4163]: message = receive push data(<a1018b3b 02000000 00000000 00000000 00000000>),with command(a1), from characteristic(8A82)
Feb 20 14:19:49 i6S lifesensehealth1_1[4163]: message = receive randomnumber (37456641)
Feb 20 14:19:49 i6S lifesensehealth1_1[4163]: message = next step is :operating_receive_random_number
Feb 20 14:19:49 i6S lifesensehealth1_1[4163]: message = next step is :operating_write_xor_results
Feb 20 14:19:49 i6S lifesensehealth1_1[4163]: message = write command with data:<200eff57 c5>
Feb 20 14:19:49 i6S lifesensehealth1_1[4163]: message = write command data((null)) to characteristic(8A81)
Feb 20 14:19:49 i6S lifesensehealth1_1[4163]: message = write success with status - operating_write_xor_results
Feb 20 14:19:49 i6S lifesensehealth1_1[4163]: message = next step is :operating_write_utc_time
Feb 20 14:19:49 i6S lifesensehealth1_1[4163]: message = write command with data:<02033b8b 0b>
Feb 20 14:19:49 i6S lifesensehealth1_1[4163]: message = write command data((null)) to characteristic(8A81)
Feb 20 14:19:49 i6S lifesensehealth1_1[4163]: message = write success with status - operating_write_utc_time
Feb 20 14:19:49 i6S lifesensehealth1_1[4163]: message = next step is :operating_write_disconnect
Feb 20 14:19:49 i6S lifesensehealth1_1[4163]: message = write command with data:<22>
Feb 20 14:19:49 i6S lifesensehealth1_1[4163]: message = write command data((null)) to characteristic(8A81)
Feb 20 14:19:49 i6S lifesensehealth1_1[4163]: message = write success with status - operating_write_disconnect
Feb 20 14:19:49 i6S lifesensehealth1_1[4163]: message = next step is :operating_uploaded_results_process

Answer 1

You need to write some data to a certain characteristic first. In my case that characteristic was 8A81. I wrote a byte array of a UTC Time code I generated through an algorithm that I cannot publish here. Try writing any 5-byte long array/char to it and see what happens (For example: [1,1,1,1,1])