Siemens SIMATIC S7 Manual page 37

db_x
start_x
end_x
ENQ_ENR
LICENSED
BUSY
OPEN MODBUS / TCP communication via CP343-1 and 443-1
2XV9450-1MB00; Manual edition 4.3
The parameter db_x defines the number of the data block in which the
consecutively defined MODBUS registers or bits are mapped.
0 cannot be used as DB number since it is reserved for system functions.
db_x
DB-Number
Start_x specifies the first register or bit address, which is stored in the data
element 0 of the DB. End_x defines the last MODBUS address.
When accessing registers, the number of the data element of the S7 DB in
which the last register is mapped, can be calculated with the following
formula:
DBW number = (end_x – start_x) ∗ 2
When accessing coils or inputs, the number of the data element of the S7 DB
in which the last bit is mapped, can be calculated with the following formula:
DBB number = (end_x – start_x + 7) / 8
The defined memory areas must not overlap. The parameter end_x must not
be smaller than start_x. In case of an error occurring, the initialization of the
FB is stopped with an error. When start_x is equal to end_x, one Modbus
address (1 register or 1 bit) is allocated.
In section 5.3 you can find an example of the mapping of the MODBUS
addresses to S7 memory areas.
Operating mode "CP is client":
The data transfer is initiated with a TRUE signal at the input. The request
telegram is generated with the values of the input parameters UNIT,
DATA_TYPE, START_ADDRESS, LENGTH and WRITE_READ. A new
request may only be initiated, when the previous one is ended with DONE or
ERROR.
Operating mode "CP is server":
The FB is activated with a TRUE signal at the input. Telegrams from the client
can be received. With a FALSE signal at the input data is received from the
CP and discarded.
If this output is TRUE, then the Modbus block is licensed on this CPU. If the
output is FALSE, none or a faulty license string was typed in. You can find
further information in section "Licensing".
If this output is TRUE, one of the functions AG_SEND or AG_RECV is
running.
Function Block MODBUSCP
1 - 65535 (W#16#0001 to W#16#FFFF)
5-9