Guralp Systems CMG-DM24 - Manual (Page 112)

CMG-DM24
Stream ID should match the 4-byte stream ID of the last received
block. Each system is able to identify its own ACK or NACK by
matching this ID. 2-byte systems only match the least significant byte
and ignore the rest.
For a NACK message, Block number specifies which block number to
rewind to.
For an ACK message, the third byte is either unused (0), or 0x13 (Ctrl-
S) to switch to command mode.
DM24 digitiser units transmit data as complete blocks become
available, without any flow control apart from ACK and NACK.
112 Issue N