Siemens SIMATIC S7 System Manual page 539

Example
Consider the following received hexadecimal coded message: "68 10 aa 68 bb 10 aa 16"
and the configured start sequences shown in the table below. Start sequences begin to be
evaluated when the first 68H character is successfully received. Upon successfully receiving
the fourth character (the second 68H), then start condition 1 is satisfied. Once the start
conditions are satisfied, the evaluation of the end conditions begins.
The start sequence processing can be terminated due to various parity, framing, or inter-
character timing errors. These errors result in no received message, because the start
condition was not satisfied.
Table 12- 16 Start conditions
Start condition First Character
1 68H
2 10H
3 dcH
4 e5H
End conditions for the RCV_PTP instruction
The end of a message is determined by the specification of end conditions. The end of a
message is determined by the first occurrence of one or more configured end conditions.
The section "Message end conditions" in the topic "Configuring receive parameters
(Page 553)" describes the end conditions that you can configure in the RCV_CFG
instruction.
You can configure the end conditions in either the properties of the communication interface
in the device configuration, or from the RCV_CFG instruction. Whenever the CPU transitions
from STOP to RUN, the receive parameters (both start and end conditions) return to the
device configuration settings. If the STEP 7 user program executes RCV_CFG, then the
settings are changed to the RCV_CFG conditions.
S7-1200 Programmable controller
System Manual, 11/2011, A5E02486680-05
First Character +1 First Character +2
xx xx
aaH xx
aaH xx
xx xx
105BCommunication processor
12.3 Point-to-Point (PtP) communication
First Character +3
68H
xx
xx
xx
First Character +4
xx
xx
xx
xx
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