尝试订阅CBCharacteristic时，不允许写入错误

Question

I'm developing an App that communicates with a Bluetooth LE device and so I'm using CoreBluetooth to do this.

The peripheral I'm using exposes 1 service which has two characteristics a serial port FIFO characteristic which supports indication/notification/write/write no response; and a serial port credit characteristic which supports write.

From my reading I gather that the characteristic that I need to subscribe to is the FIFO one but when I call [_connectedPeripheral setNotifyValue:YES forCharacteristic:characteristic]; I get a Writing is not permitted error.

I checked the characteristic's properties and it has the Read , WriteWithoutResponse , Notify , and Indicate properties.

This is the first time I've used CoreBluetooth and I'm a bit of a bluetooth noob so it's probably something obvious I've overlooked but any help would be much appreciated.

EDIT: Here's the code:

#define SDL440S_SERVICE @"2456E1B9-26E2-8F83-E744-F34F01E9D701"
#define SDL440S_CHARACTERISTIC @"2456E1B9-26E2-8F83-E744-F34F01E9D703"

- (void)centralManager:(CBCentralManager *)central didConnectPeripheral:(CBPeripheral *)peripheral
{
    NSLog(@"Did connect %@", peripheral.name);
    // _connectedPeripheral is a property...
    if (_connectedPeripheral != peripheral) {
        _connectedPeripheral = peripheral;
    }
    _connectedPeripheral.delegate = self;
    [_connectedPeripheral discoverServices:@[[CBUUID UUIDWithString:SDL440S_SERVICE]]];
}

#pragma mark - <CBPeripheralManager>

- (void)peripheral:(CBPeripheral *)peripheral didDiscoverServices:(NSError *)error
{
    if (peripheral == _connectedPeripheral) {
        for (CBService *service in _connectedPeripheral.services) {
            if ([service.UUID.UUIDString isEqualToString:SDL440S_SERVICE]) {
                [_connectedPeripheral discoverCharacteristics:@[[CBUUID UUIDWithString:SDL440S_CHARACTERISTIC]] forService:service];
            }
        }
    }
}

- (void)peripheral:(CBPeripheral *)peripheral didDiscoverCharacteristicsForService:(CBService *)service error:(NSError *)error
{
    for (CBCharacteristic *characteristic in service.characteristics) {
        if ([characteristic.UUID.UUIDString isEqualToString:SDL440S_CHARACTERISTIC]) {
            // Subsribe to characteristic
            [_connectedPeripheral setNotifyValue:YES forCharacteristic:characteristic];
        }
    }
}

// Getting values

- (void)peripheral:(CBPeripheral *)peripheral didUpdateNotificationStateForCharacteristic:(CBCharacteristic *)characteristic error:(NSError *)error
{
    if (error) {
        NSLog(@"Error changing note state for char: %@.\nError: %@", characteristic, error.localizedDescription);
    }
}

Answer 1

For future reference if anyone comes across this problem, it was to do with the device's credit characteristic.

The service which I was interested in exposed two characteristics a credit characteristic and a FIFO characteristic. (I understand that a credit characteristic is standard issue.) The problem was that I had to write the maximum amount of data that i wanted to receive to the credit characteristic before i could then receive and write data to the FIFO characteristic.

Ie:

unsigned char buf[1] = {/*relevant data here...*/};
NSData *data = [NSData dataWithBytes:buf length:1];
[peripheral writeValue:data forCharacteristic:creditCharacteristic type:CBCharacteristicWriteWithoutResponse];