With Profibus, a "token" is constantly transmitted through the system, from which the respective subscribers take the data applicable to them and to which the subscribers write the data for the master, each received fragment must be acknowledged with ,,FRAG ACK'' and the D-Len byte of the received fragment.
When a fragmented message is sent to the master, each fragment must be acknowledged by the master with "FRAG ACK" and the D-Len byte of the received fragment, so that the module can dispatch the next module.
If a fragmented message is sent by the master to the module, the next fragment can only be dispatched when the module has acknowledged the receipt of the previous one (with "FRAG ACK" and the D-Len byte of the received fragment). The last fragment must not be acknowledged.