Siemens Simatic S7-200 System Manual page 159

The protocols support 127 addresses (0 through 126) on a network. There can be up to 32
master devices on a network. All devices on a network must have different addresses in
order to be able to communicate. SIMATIC programming devices and PCs running
STEP 7-Micro/WIN have the default address of 0. Operator panels such as the TD 200, OP3,
and the OP7 default to address 1. The programmable controllers have the default address of
2. The DP port on the CPU 215 has the default address of 126.
PPI Protocol
PPI is a master/slave protocol. In this protocol the master devices (other CPUs, SIMATIC
programming devices, or TD 200s) send requests to the slave devices and the slave devices
respond. Slave devices do not initiate messages, but wait until a master sends them a
request or polls them for a response. All S7-200 CPUs act as slave devices on the network.
Some S7-200 CPUs can act as master devices while they are in RUN mode, if you enable
PPI master mode in the user program. (See the description of SMB30 in Appendix D.) Once
PPI master mode has been enabled, you can read from or write to other CPUs by using the
Network Read (NETR) and Network Write (NETW) instructions. See Chapter 10 for a
description of these instructions. While acting as a PPI master, the S7-200 CPU still
responds as a slave to requests from other masters.
PPI has no limit on how many masters can communicate to any one slave CPU, but there
can be no more than 32 masters on a network.
MPI Protocol
MPI may be either a Master/Master protocol or a Master/Slave protocol. Exactly how the
protocol operates is based on the type of device. If the destination device is an S7-300 CPU,
then a master/master connection is established because all S7-300 CPUs are network
masters. If the destination device is an S7-200 CPU, then a master/slave connection is
established because the S7-200 CPUs are slave devices.
MPI always establishes a connection between the two devices communicating with each
other. A connection is like a private link between the two devices. Another master cannot
interfere with a connection established between two devices. A master can establish a
connection to use for a short period of time, or the connection can remain open indefinitely.
Because the connections are private links between devices and require resources in the
CPU, each CPU can only support a finite number of connections. Table 9-2 lists the number
and type of MPI connections supported by each S7-200 CPU. Each CPU reserves some of
its connections for SIMATIC programming devices and operator panels. The reserved
connection for a SIMATIC programming device or PC running STEP 7-Micro/WIN ensures
that the user is always able to attach at least one SIMATIC programming device to the CPU
and gain access to the CPU. Some CPUs also reserve a connection for an operator panel.
These reserved connections cannot be used by other types of master devices (such as
CPUs).
S7-200 Programmable Controller System Manual
C79000-G7076-C230-02
Network Communications and the S7-200 CPU
9-3