Encoder Value Detection - Siemens SIMATIC ET 200S Operating Instructions Manual

4.6.2

Encoder Value Detection

Description
The absolute encoder transfers its encoder values in message frames to the 1SSI. The
transmission of message frames is initiated by the 1SSI. The following alternatives are
available for encoder value detection:
● Free-wheeling encoder value detection
● Synchronous encoder value detection
● Isochronous encoder value detection
You can set free-wheeling or synchronous encoder value detection in HW Config with the
"Detection" parameter. This parameter only works in non-isochronous mode.
The detection of the encoder value will be isochronous when the 1SSI is in isochronous
mode. In this case, the "Detection" parameter is not evaluated.
The following table shows these connections:
Table 4- 2 Encoder Value Detection
Mode
Non-isochronous mode
Isochronous mode
Free-Wheeling Encoder Value Detection
With free-wheeling encoder value detection, you obtain maximum accuracy with the latch
function.
The 1SSI initiates the transmission of a message frame each time the assigned monoflop
time elapses.
The 1SSI processes the detected encoder value asynchronously to these free-wheeling
message frames in the cycle of the update rate.
This results in encoder values of various ages with the free-wheeling detection of encoder
values. The difference between maximum and minimum age is the jitter.
Synchronous Encoder Value Detection
With synchronous encoder value detection, you obtain maximum accuracy with encoder
value detection.
The 1SSI initiates the transmission of a message frame in the cycle of its update rate.
The 1SSI processes the transmitted encoder value synchronously to its update rate.
Isochronous Encoder Value Detection
Isochronous encoder value detection is carried out automatically when the equidistant bus
cycle is activated in the DP master system and the DP slave is synchronized to the bus
cycle.
The 1SSI initiates the transmission of a message frame in every bus cycle at time T
as the configured maximum encoder sampling rate does not result in a reduction.
The 1SSI processes the transmitted encoder value isochronously to the bus cycle.
Technological Functions
Operating Instructions, 06/2010, A5E00124867-07
"Detection" Parameter
Free-wheeling
Synchronous
- (irrelevant)
4.6 Functions of the 1SSI
Encoder value detection
Free-wheeling encoder value detection
Synchronous encoder value detection
Isochronous encoder value detection
1SSI
, as long
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