Specifications

Setting Up Communications Hardware and Network Communications
7-29
S7-200 Programmable Controller System Manual
C79000-G7076-C233-01
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 C.) 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 Section 9.16, SIMATIC Communications Instructions in
Chapter 9 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. Every CPU
supports four connections. Each CPU reserves two of its connections; one for a
SIMATIC programming device or PC, and one for operator panels. The reserved
connection for a SIMATIC programming device or PC enables you to always attach
at least one SIMATIC programming device or PC to the CPU. The CPUs also
reserve a connection for an operator panel. These reserved connections cannot be
used by other types of master devices (such as CPUs).
The S7-300 and S7-400 CPUs can communicate with the S7-200 CPUs by
establishing a connection on the non-reserved connections of the S7-200 CPU.
The S7-300s and S7-400s can read and write data to the S7-200s using the XGET
and XPUT instructions (refer to your S7-300 or S7-400 programming manuals).