Siemens SIMATIC S7-200 System Manual page 250

S7-200 Programmable Controller System Manual
As shown in Table 7-13, the S7-200 CPU or EM 277 provide a specific number of connections.
Each port (Port 0 and Port 1) of an S7-200 CPU supports up to four separate connections. (This
allows a maximum of eight connections for the S7-200 CPU.) This is in addition to the shared PPI
connection. An EM 277 supports six connections. Each port reserves one connection for a
programmer and one connection for an operator panel (OP or TP). The remaining connections are
available for general use.
Table 7-13 Capabilities of the S7-200 CPU and EM 277 Modules
Connection Point
S7-200 CPU
EM 277 Module
1
If a CP card is used to connect STEP 7- -Micro/WIN to the S7-200 CPU through Port 0 or Port 1, you can select
either MPI or DP PROFIBUS profiles only when the S7-200 device is configured as a slave.
2 In addition to the PROFIBUS connection.
Working with Complex Networks
For the S7-200, complex networks typically have multiple S7-200 masters that use the Network
Read (NETR) and Network Write (NETW) instructions to communicate with other devices on a PPI
network. Complex networks typically present special problems that can block a master from
communicating with a slave.
If the network is running at a lower baud rate (such as 9.6 kbaud or 19.2 kbaud), then each master
completes the transaction (read or write) before passing the token. At 187.5 kbaud, however, the
master issues a request to a slave and then passes the token, which leaves an outstanding
request at the slave.
Figure 7-33 shows a network with potential communications conflicts. In this network, Station 1,
Station 2, and Station 3 are masters, using the Network Read or Network Write instructions to
communicate with Station 4. The Network Read and Network Write instructions use PPI protocol
so all of the S7-200s share the single PPI connection in Station 4.
In this example, Station 1 issues a request to
Station 4. For baud rates above 19.2 kbaud,
Station 1 then passes the token to Station 2. If
Station 2 attempts to issue a request to Station 4,
the request from Station 2 is rejected because the
request from Station 1 is still present. All requests
to Station 4 will be rejected until Station 4 completes
the response to Station 1. Only after the response
h has been completed can another master issue a b
request to Station 4.
To avoid this conflict for the communications port on
Station 4, consider making Station 4 the only master
on the network, as shown in Figure 7-34. Station 4
then issues the read/write requests to the other
S7-200s.
Not only does this configuration ensure that there is
no conflict in communications, but it also reduces
the overhead caused by having multiple masters the overhead caused by having multiple masters
and allows the network to operate more efficiently.
236
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
l t d th t
STEP 7- - Micro/WIN Protocol Profile
Connections
Selections
4 PPI, PPI Advanced, MPI, and PROFIBUS
4 PPI, PPI Advanced, MPI, and PROFIBUS
2
6 per module PPI Advanced, MPI, and PROFIBUS
Station 1 Master
i
Figure 7-33 Communications Conflict
Station 1 Slave
Figure 7-34 Avoiding Conflict
Station 2 Master Station 3 Master
Station 4 Slave
Station 2 Slave Station 3 Slave
Station 4 Master
1
1