Specifications

Setting Up Communications Hardware and Network Communications
7-44
S7-200 Programmable Controller System Manual
C79000-G7076-C233-01
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 9.16,
SIMATIC Communications Instructions in Chapter 9. If you send messages using
these instructions, you can use the formula shown in Figure 7-29 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.
Token hold time (T
hold
) = (128 overhead + n data char) < 11 bits/char < 1/baud rate
Token rotation time (T
rot
) = T
hold
of master 1 + T
hold
of master 2 + . . . + T
hold
of master m
where n is the number of data characters (bytes)
and m is the number of masters
Solving for the token rotation time using the example shown above, where each of the six
masters has the same token hold time, yields:
T (token hold time) = (128 + 4 char) < 11 bits/char < 1/9600 bit times/s
= 151.25 ms/master
T (token rotation time) = 151.25 ms/master < 6 masters
= 907.5 ms
(One “bit time” equals the duration of one signaling period.)
Figure 7-29 Formulas for Token Hold Time and Token Rotation Time, Using NETR and
NETW