Siemens SIMATIC S7 System Manual page 820

Special memory (SM) and system symbol names
D.16 SMB86-SMB94 and SMB186-SMB194: Receive message control
You use the bits of the message control byte define the criteria for identifying the message.
The message control byte also includes the start of message and end of message criteria.
To determine the start of a message, either of two sets of logically ANDed start-of-message
criteria must be true and must occur in sequence. "In sequence" means one of the following:
● Start character follows idle line
● Start character follows break
A logical OR of the end-of-message criteria determines the end of a message. The
equations for start and end criteria are as follows:
Start of message = (il AND sc) OR (bk AND sc)
End of message = ec OR tmr OR maximum character count reached
820
System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART