Siemens SIMATIC S7-1200 Reference Manual page 65

Basic instructions Extended instructions Technology Communication
S7 communication
Definition: SIMATIC-homogeneous data exchange between SIMATIC CPUs via PROFIBUS or PROFINET/Industrial
Ethernet.
With S7 communication, you connect existing S7-300/400 to S7-1200/1500 or migrate existing systems to S7-1200/1500.
Recommendation: Use open communication for data exchange between S7-1200/1500 and thus the possibilities of common
Ethernet standards.
Coordinated data transmission with BSEND and BRCV
BSEND sends data to an instruction of the type BRCV in a partner controller. Since BSEND and BRCV coordinate the data
transfer, BSEND/BRCV transport the largest amount of data of all the configured S7 connections. BSEND segments the data
area to be sent and sends each segment individually to the partner. BRCV acknowledges the acceptance of the sent segment.
When BRCV has acknowledged the receipt of the complete data area, you can start a new send job BSEND.
Uncoordinated data transmission with USEND and URCV
USEND sends data to an instruction of the type URCV in a partner controller. URCV does not acknowledge the receipt of the
data. The data transfer is not coordinated with the partner controller. This means that USEND can overwrite received data
before URCV has written all the data to the target area. If USEND overwrites data, the receiver outputs an error message.
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