Figure 39: Number Of Free Slots; Figure 40: Acknowledgement Of A Diagnostic Message - WAGO I/O-SYSTEM 750 Manual

100 Function Description
Max. 11 unacknowledged messages are possible!
Note that unacknowledged messages cannot be overwritten.
If you have no unacknowledged messages, then after 11 entries having
unacknowledged messages, all slots will be filled except for the last. In this
case, an EMCY message, "History memory full", will be generated and sent
to the master. This EMCY message will be recorded in the last free slot
instead of a final entry for the last occurring error and SI5 bit 5 set.

Figure 39: Number of Free Slots

Subindex 3 contains the subindex number of the last acknowledged message. If
a new value is written to SI3, all messages up to the written subindex number are
acknowledged.

Figure 40: Acknowledgement of a Diagnostic Message

Manual
Version 1.2.1
750-354/000-001 EtherCAT® Fieldbus Coupler, ID Switch
WAGO-I/O-SYSTEM 750