Rockwell Automation 1769-SM2 Compact I/O DSI/Modbus Communication Module - Manual (Page 160)

9-8 Troubleshooting
52 Msg Ctrl Open The module has begun receiving Client-Server Control
messages (the Client-Server Control Timeout was previously
set to a non-zero value).
53 Msg Ctrl Close The device sending Client-Server Control messages to the
module has set the Client-Server Control Timeout to a value of
zero.
54 Msg Ctrl
Timeout The module has not received a Client-Server Control message
for longer than the established timeout period.
Table 9.C Module Events (Continued)
Code Event Description