Siemens SIMATIC S7 Manual page 35

ENQ_ENR
DISCONNECT
REG_KEY
BUSY
CONN_
ESTABLISHED
MODBUS / TCP communication via the integrated PN interface of the CPU
2XV9450-1MB02; Manual edition 2.1
Operating mode S7 is Client:
The data transfer is initiated with a TRUE edge at the input. The request
telegram is generated with the values of the input parameters UNIT,
DATA_TYPE, START_ADDRESS, LENGTH, TI and WRITE_READ. A new
request may only be initiated when the previous one is ended with
DONE_NDR or ERROR.
If the connection is not established (CONN_ESTABLISHED = FALSE), the
connection establishment is carried out first and then the data transfer takes
places.
Operating mode S7 is Server:
The FB is activated with a TRUE signal at the input. Telegrams from the client
can be received.
If the connection is not established (CONN_ESTABLISHED = FALSE), the
connection establishment is activated.
When the parameter ENQ_ENR turns from TRUE to FALSE, the connection
may be terminated depending on the parameter DISCONNECT.
With a FALSE signal at the input and an established connection, data is
received from the client and discarded.
In the operating mode S7 is client, the parameter DISCONNECT = TRUE
indicates that the connection is terminated after the completed data transfer.
In the operating mode S7 is server, the parameter DISCONNECT = TRUE
indicates that the connection is terminated when the parameter ENQ_ENR is
set to FALSE.
This parameter is a runtime parameter and can be set optionally according to
your requirements.
The block MODBUSPN must be licensed for each CPU individually to permit
a correct program sequence.
With the registration key REG_KEY the block MODBUSPN is licensed and
the Modbus communication runs without any restraint.
You can find further information in section 5 "Licensing".
If this output is TRUE, one of the T functions TCON, TDISCON, TSEND or
TRCV is running.
CONN_ESTABLISHED indicates that a connection to the communication
partner is established and data can be transferred.
If CONN_ESTABLISHED is set to FALSE, the connection to the
communication partner is not established.
Function Block MODBUSPN
6-9