Unable to send notification from central

Question

I have adopted the S120/S110 multilink example to the following: central.zip and peripheral main.c 
 I want to send a notification from the central to the peripheral and back (the second notification was already implemented. 
 I know the error is in the line below, because when I comment it the connection remains active. Otherwise the connection is dropped and the peripheral request a new one. 
 I think I'm not using the correct value_handle . But where can I find the correct value? 
 I'm using SDK8 with S110v8 and S120v2 in Keil (on two nRF51 DK's). 
 Update 
 New code (entire project). Now I can send a write command as suggested by @Nathan. But I can send a notification from the peripheral to the central and a write command from central to peripheral. But when I first send a write command followed by a notification the peripheral crashes. Why? 
 Also, I used the rssi as defined in the SDK, but the received data is not really realistic (higher power than received). Is there a fix that keeps the current implementation or should I change everything to something as suggested in this post 
 Finally, any suggestions as to why my code is already to large for de nRF51 DK? When I define TRILATERATION Keil complains there isn't enough memory on the device.

Ulrich Myhre · Accepted Answer

I took a brief look at your code, and could not see that you guard against sending notifications before the peer has enabled them - nor could I find any trace in the peripheral where it enabled them. Are you sure that this is not what is happening? The central is calling notif_enable() during initial DB discovery, which is why the peripheral is able to notify its values. The peripheral does not do anything like this, which makes you unable to send notifications. 
 When debugging things like this, you can set a breakpoint in the application error handler function and catch the errors when they happen. You should then see what errorcode that was returned and the complete callstack, which helps you search for similar problems here on DevZone - or give an even more descriptive answer. 
 I am also not completely sure what you are trying to accomplish here, but it seems to me that you just want to send data back and forth every now and then? And perhaps save it? If that is the case, normal write commands or write requests might be the easiest way. Even though notifications are able to transmit data, their purpose is to notify a GATT client that a value in the GATT server has changed, while simultaneously giving (parts of) the new data. But to not spam down every device with updates, you have to explicitly subscribe to each characteristic you want notifications about, and keep track of these preferences between connections (if bonded). This is a bit overkill for just sending data. 
 @Nathan's answer: Both the central and the peripheral has a GATT server and a GATT client. They are completely role-agnostic. The only differences between our central and peripheral GATT implementations are the number of packet buffers (less on the central due to many links), that PPCP only exists for peripheral, and the fact that a characteristic with a CCCD has an individual CCCD for each link on the central. This means that you have to keep track of the connection handle in addition to the state of every CCCD, and look them up before sending. Any knowledge of GATT you have from S110 should apply to S120 if you keep these differences in mind.

Unable to send notification from central

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