前几天从西门子的FAQ上看到了下面这篇文章
Sampling interval and number of contollers in a S7 CPU
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Entry ID: 17624666
Date: 11/25/2003
QUESTION:
How can I estimate the sampling interval and the largest possible number of controllers in a S7 CPU?

ANSWER:
If you want to operate a larger number of controllers in a S7 CPU, it is necessary to estimate at least approximately the sampling interval of the controllers, the runtimes of the function block and the memory space requirement.

The regulation function blocks are called 3 times in the time interrupt. Equidistant sampling times or control cycles (e.g. 5 ms, 20 ms, 100 ms) are implemented by this. Depending on the number of controllers it is reasonable, to edit several controllers in an OB call, in order to get shorter sampling intervals. The more controllers are edited in an OB call, the bigger are, however, the reimpacts on OB1 (cycle load).
Besides the number of calls, the runtimes of the FBs still have to be taken into account (reimpacts on OB1).

Example:
The runtimes of the FBs for a control system are 5 ms. In OB 3x two control systems are edited one by one (time interrupt of the OB =20 ms); OB1 is extended every 20 ms by 10 ms.

You can obtain an overview, with the following empirical formulas:

Storage space:
FBs + (Number of controllers) * (Lenght of the instance-DBs)

Minimal sampling interval:

S7-300: 20 ms *(Number of controllers)
S7-400: 5 ms *(Number of controllers)
Examples for the sampling interval:

50 controllers are intended to be operated in a CPU S7 300. If in an OB 3x-cycle one controller is edited, the result is a sampling time of 1 second. With two calls per OB cycle, the sampling interval halves to 500 ms (with higher OB1 strain). Vice versa, you can determine the number of controllers based on the sampling interval.
80 controllers are intended to be operated in a CPU S7 400. For one controller call, the result is 400 ms, for two calls it is 200 ms.
The regulation option packages provide a call distributor, which determines the call order and frequency in a convenient way for the time interrupt OBs 3x. For the integrated controllers, this is up to the responsibility of the user.