Sending Messages; Token Rotation Time - Siemens SIMATIC S7-200 System Manual

Setting Up Communications Hardware and Network Communications
In this configuration, the TD 200 (station 3) communicates with the CPU 222
(station 2), TD 200 (station 5) communicates with CPU 222 (station 4), and so on.
Also, CPU 224 (station 6) is sending messages to stations 2, 4, and 8, and
CPU 224 (station 8) is sending messages to stations 2, 4, and 6. In this network,
there are six master stations (the four TD 200 units and the two CPU 224 modules)
and two slave stations (the two CPU 222 modules).

Sending Messages

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.

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 S7-200 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 CPUs by using the Network Read (NETR) and Network Write (NETW)
instructions with the CPU. See the description of these instructions in Section 9.15,
SIMATIC Communications Instructions in Chapter 9. If you send messages using
these instructions, you can use the formula shown in Figure 7-30 to calculate the
approximate token rotation time when the following assumptions are true:
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 communication buffer in the CPU.
There is no CPU that has a scan time longer than about 10 ms.
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S7-200 Programmable Controller System Manual
A5E00066097-02