Selecting The Communications Protocol For Your Network - Siemens SIMATIC S7-200 System Manual

S7-200 Programmable Controller System Manual

Selecting the Communications Protocol for Your Network

The following information is an overview of the protocols supported by the S7-200 CPUs.
Point-to-Point Interface (PPI)
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Multi-Point Interface (MPI)
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PROFIBUS
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Based on the Open System Interconnection (OSI) seven-layer model of communications
architecture, these protocols are implemented on a token ring network which conforms to the
PROFIBUS standard as defined in the European Standard EN 50170. These protocols are
asynchronous, character-based protocols with one start bit, eight data bits, even parity, and one
stop bit. Communications frames depend upon special start and stop characters, source and
destination station addresses, frame length, and a checksum for data integrity. The protocols can
run on a network simultaneously without interfering with each other, as long as the baud rate is the
same for each protocol.
Ethernet is also available for the S7-200 CPU with expansion modules CP243--1 and CP243--1 IT.
PPI Protocol
PPI is a master-slave protocol: the master devices
send requests to the slave devices, and the slave
devices respond. See Figure 7-7. Slave devices do
not initiate messages, but wait until a master sends
them a request or polls them for a response.
Masters communicate to slaves by means of a
shared connection which is managed by the PPI
protocol. PPI does not limit the number of masters
that can communicate with any one slave; however,
you cannot install more than 32 masters on the you cannot install more than 32 masters on the
network.
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.) After enabling PPI
master mode, you can use the Network Read or the Network Write instructions to read from or
write to other S7-200s. While the S7-200 is acting as a PPI master, it still responds as a slave to
requests from other masters.
PPI Advanced allows network devices to establish a logical connection between the devices. With
PPI Advanced, there are a limited number of connections supplied by each device. See Table 7-3
for the number of connections supported by the S7-200.
All S7-200 CPUs support both PPI and PPI Advanced protocols, while PPI Advanced is the only
PPI protocol supported by the EM 277 module.
Table 7-3 Number of Connections for the S7-200 CPU and EM 277 Modules
Module
S7-200 CPU
EM 277 Module
214
Baud Rate
Port 0 9.6 kbaud, 19.2 kbaud, or 187.5 kbaud
Port 1 9.6 kbaud, 19.2 kbaud, or 187.5 kbaud
9.6 kbaud to 12 Mbaud
STEP 7- -Micro/WIN:
Master
Figure 7-7 PPI Network
Connections
4
4
6 per module
S7-200
HMI: Master