Siemens SIMATIC S7-200 System Manual page 248

S7-200 Programmable Controller System Manual
Refer to the Programming Tips on the documentation CD for a discussion about token rotation.
See Tip 42.
Programming
Tips
CPU 222
Station 2
Figure 7-31 Example of a Token-Passing Network
In order for a master to send a message, it must hold the token. For example: When station 3 has
the token, it initiates a request message to station 2 and then it passes the token to station 5.
Station 5 then initiates a request message to station 4 and then passes the token to station 6.
Station 6 then initiates a message to station 2, 4, or 8, and passes the token to station 7. This
process of initiating a message and passing the token continues around the logical ring from
station 3 to station 5, station 6, station 7, station 8, station 9, and finally back to station 3. The
token must rotate completely around the logical ring in order for a master to be able to send a
request for information. For a logical ring of six stations, sending one request message per token
hold to read or write one double-word value (four bytes of data), the token rotation time is
approximately 900 ms at 9600 baud. Increasing the number of bytes of data accessed per
message or increasing the number of stations increases the token rotation time.
The token rotation time is determined by how long each station holds the token. You can
determine the token rotation time for your multiple-master network by adding the times that each
master holds the token. If the PPI master mode has been enabled (under the PPI protocol on your
network), you can send messages to other S7-200s by using the Network Read and Network
Write instructions with the S7-200. If you send messages using these instructions, you can use
the following formula to calculate the approximate token rotation time, based on the following
assumptions: each station sends one request per token hold, the request is either a read or write
request for consecutive data locations, there is no conflict for use of the one communications
buffer in the S7-200, and there is no S7-200 that has a scan time longer than about 10 ms.
Token hold time (T
Token rotation time (T
where
The following equations calculate the rotation times (one "bit time" equals the duration of one
signaling period) for the example shown in Figure 7-31:
T (token hold time)
T (token rotation time)
Tip
SIMATIC NET COM PROFIBUS software provides an analyzer to determine network
performance.
234
CPU 222 CPU 224 CPU 224
Station 4 Station 6 Station 8
) = (128 overhead + n data char) x 11 bits/char x 1/baud rate
hold
) = T of master 1 + T
rot hold
n is the number of data characters (bytes)
m is the number of masters
= (128 + 4 char) x 11 bits/char x 1/9600 bit times/s
= 151.25 ms per master
= 151.25 ms per master
= 907.5 ms
TD 200 TD 200
Station 9 Station 7
of master 2 + . . . + T
hold hold
6 masters
≦
TD 200 TD 200
Station 5 Station 3
of master m