Siemens SIMATIC S7 Manual page 30

Initiate Request
S7 is Client
Activation of the
Function Block
S7 is Server
MODBUS / TCP communication via the integrated PN interface of the CPU
2XV9450-1MB02; Manual edition 2.1
A rising edge at the trigger input ENQ_ENR initiates a request. Depending on
the input parameters UNIT, DATA_TYPE, START_ADDRESS, LENGTH, TI
and WRITE_READ, a MODBUS request telegram is generated and sent to
the partner station via the TCP/IP connection. The client waits for the
parameterized monitoring time RECV_TIME for a response from the server.
When the monitoring time elapses (no response from the server), the
activated request is terminated with an error. A new request can be initiated.
After the receipt of the response telegram, a validity check is carried out. If the
result is positive, the necessary actions are taken and the request is
terminated without error. The output DONE_NDR is set. When an error is
recognized during verification, the request is terminated with an error, the
ERROR bit is set and an error number is returned at the output STATUS.
With the signal TRUE at the trigger input ENQ_ENR, the FB is ready to
receive a request telegram from the client. The server remains passive and
waits for a telegram from the client. The received telegram is verified. If the
verification result is positive, the response telegram is sent. The completed
transmission is reported to the user by setting the DONE_NDR bit. At this
point, the completed function is indicated at the outputs UNIT, DATA_TYPE,
START_ADDRESS, LENGTH, TI and WRITE_READ.
An erroneous request telegram causes an error message and the ERROR bit
is set. The error number is returned in STATUS_MODBUS. The request of
the client is not answered.
Function Block MODBUSPN
6-4