Siemens Simatic S7-200 System Manual page 161

User-Defined Protocols (Freeport)
Freeport communications is a mode of operation through which the user program can control
the communication port of the S7-200 CPU. Using Freeport mode, you can implement
user-defined communication protocols to interface to many types of intelligent devices.
The user program controls the operation of the communication port through the use of the
receive interrupts, transmit interrupts, the transmit instruction (XMT) and the receive
instruction (RCV). The communication protocol is controlled entirely by the user program
while in Freeport mode. Freeport mode is enabled by means of SMB30 (port 0) and SMB130
(port 1) and is only active when the CPU is in RUN mode. When the CPU returns to STOP
mode, Freeport communications are halted and the communication port reverts to normal
PPI protocol operation. See Section 10.14 for information about using the Freeport mode.
Figure 9-1
S7-200 Programmable Controller System Manual
C79000-G7076-C230-02
Network Communications and the S7-200 CPU
S7-300 with
CPU 315-2 DP
CPU 215 Connected to an S7-300 CPU and a Programming Device by Means of the
DP Port
SIMATIC programming device
CPU 215
9-5