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S5-115U Manual
Sampling
The 460 and 465 modules offer two methods of sampling the analog value:
•
Cyclic sampling and
•
Selective sampling
The 466 module implements only cyclic sampling because of its high speed.
Cyclic Sampling
The modules's processor decodes all inputs.
However, there are differences between the individual modules.
For example, the amount of time that elapses before a measured value is updated depends on the
number of input channels. The time required for decoding depends on the input value. In the case
of the 460 analog input module, when V
decoding takes 60 ms.
Module
460
Channels
Scan Time*
480 ms
* Nominal value applied to all input s
In the case of the 460/465 modules, the digitized measured values are stored in the circulating
buffer under the channel address (the high-order byte under address n, the low-order byte under
address n+1), and can be read out from the buffer whenever required.
Selective Sampling
Selective sampling is not possible on the 466 module.
Double addressing cannot be used for selective sampling, i.e. an address cannot be assigned to an
analog output module and an analog input module.
In the case of the 460 and 465 modules, the initiative for decoding a measured value comes from
the CPU when this function is used. The module must be accessed once with a Write command
(T PW) under the relevant channel address; the data itself is of no relevance. In this way, only the
measured value of the activated channel is decoded and the other channels are ignored. During
decoding, an activity bit is set on the data bus (A=1, see also Section 10.6). The module sets the
activity bit independently, i.e. if several channels are to be decoded using selective sampling, the
activity bit cannot be assigned to one channel! The valid digitized measured value can be read out
from two bytes once the activity bit has been reset (A=0, negative-going edge).
Repeated scanning of the activity bit loads both the bus and the CPU. This results in non-periodic
measured value acquisition when different measured values are involved, and is therefore not
desirable for PID control tasks.
A better method is time-controlled program scanning, in which certain program sections, for
instance FB13, are automatically inserted into the program every 100 ms by a time-controlled
block (OB13), thus producing a constant time base while offloading the bus and the CPU.
EWA 4NEB 811 6130-02b
=0 V, decoding takes 40 ms; when V
I
Table 10-35. Scan Times
465
8
8
480 ms
Analog Value Processing
=nominal value,
I
16
8
960 ms
2 ms
466
16
4 ms
10-53
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