Technical data

Table Of Contents
System Software for S7-300/400 System and Standard Functions - Volume 2/2
A5E00739858-01
22-1
22 Communication SFCs for Non-Configured
S7 Connections
22.1 Common Parameters of the Communication SFCs
Input Parameter REQ
The input parameter REQ (request to activate) is a level-triggered control
parameter. It is used to trigger the job (the data transfer or the connection abort):
If you call the SFC for a job that is not currently active, you trigger the job with
REQ=1. If there is no connection to the communication partner when the
communication SFC is called the first time, the connection is established before
data transfer begins.
If you trigger a job and it is not yet completed when you call the SFC again for
the same job, REQ is not evaluated by the SFC.
Input Parameter REQ_ID (only SFC 65 and SFC 66)
The input parameter REQ_ID is used to identify your send data. It is passed by the
operating system of the sending CPU to the SFC 66 "X_RCV" of the CPU of the
communication partner.
You require the REQ_ID parameter on the receiving end
If you call several SFCs 65 "X_SEND" with different parameters REQ_ID on
one send CPU and transfer the data to a communication partner.
If you use SFC 65 "X_SEND" to send data to one communication partner from
several send CPUs.
By evaluating REQ_ID you can save the received data in different memory areas.
Output Parameters RET_VAL and BUSY
The communication SFCs are executed asynchronously, this means that the
execution of a job extends over more than one SFC call. The output parameters
RET_VAL and BUSY indicate the status of the job. See also Meaning of the
Parameters REQ, RET_VAL and BUSY with Asynchronous SFCs