Siemens SIMATIC CP 440 Manual page 90

Communication Using Function Blocks
6.1 Communication Using Function Blocks
Communication between the CPU, the CP 440 and a communication partner takes
place via the function blocks and the protocols of the CP 440.
Communication Between the CPU and CP 440
The function blocks form the software interface between the CPU and the CP 440.
They must be called cyclically from the user program.
Restriction in Multicomputing Operation
In multicomputing operation only one CPU can access the CP because
communication between the CPU and CP is not multicomputing-compatible.
Communication Between the CP 440 and the Communication Partner
The transmission protocols are implemented on the CP 440. By means of the
protocol, the interface of the CP 440 is adapted to the interface of the
communication partner.
This enables you to link an S7 programmable logic controller with any
communication partner that can handle the ASCII driver and the 3964(R)
procedure.
Calling the Function Blocks in Interrupt OBs
It is not permissible to simultaneously call a function block in OB1 and in the
interrupt OB (time OBs). The reason for this is that the function block cannot be
called again if an interrupt OB triggers an interruption. If you need to call the
function block in OB1 and in the interrupt OB, you must disable the interrupts
(SFC 41) in the lower priority OB before the FB call and enable them again after
the FBs have executed.
6-2
Point-to-point connection CP 440 Installation and Parameter Assignment
A5E00057742-02