Siemens Simatic S7-200 System Manual page 241

CPU 222 CPU 222
Station 2 Station 4
Figure 7-26 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 ) = (128 overhead + n data char) x 11 bits/char x 1/baud rate
hold
Token rotation time (T ) = T
rot hold
where n is the number of data characters (bytes)
m is the number of masters
The following equations calculate the rotation times (one "bit time" equals the duration of one signaling
period) for the example shown in Figure 7-26:
T (token hold time) = (128 + 4 char) x 11 bits/char x 1/9600 bit times/s
= 151.25 ms per master
T (token rotation time) = 151.25 ms per master
= 907.5 ms
Tip
SIMATIC NET COM PROFIBUS software provides an analyzer to determine network performance.
CPU 224 CPU 224 TD 200
Station 6 Station 8 Station 9
of master 1 + T of master 2 + . . . + T
hold
x
6 masters
Communicating over a Network
TD 200 TD 200 TD 200
Station 7 Station 5 Station 3
of master m
hold
Chapter 7
7
229