Siemens SIMATIC ET 200SP - Position Input with SSI Absolute Encoder

The basics of counting, measurement and position detection
2.2 Basics of counting, measuring and position input (TM Count, TM PosInput, Compact CPU)
Counting, measurement and position detection
24 Function Manual, 12/2017, A5E32009889-AG
2.2.3.2
Position input with SSI absolute encoder
Description
You can use the TM PosInput technology modules with an SSI absolute encoder for position
detection. The technology module reads the position value via a synchronous serial interface
from the SSI absolute encoder and makes it available to the controller.
You can switch the digital outputs of the technology module exactly at defined position
values, regardless of the user program. Position input with an SSI absolute encoder does not
involve gate control. Due to system constraints, synchronization is not possible with an SSI
absolute encoder.
Gray-dual conversion
Gray-code and dual-code SSI absolute encoders are supported.
Capture (Latch)
You can configure one or both edges of a digital input that triggers a saving of the current
position value as Capture value.
Hysteresis
You can specify hysteresis for the comparison values, within which a digital output is
prevented from switching again. An encoder can come to a standstill at a specific position,
and slight movements may make the position value fluctuate around this position. If a
comparison value or a limit lies within this fluctuation range, the corresponding digital output
is switched on and off with corresponding frequency if hysteresis is not used. The hysteresis
prevents these unwanted switching operations.
Range for position value
You can specify a frame length of 10 bits to 40 bits for the SSI absolute encoder. The
configurable bit numbers of the LSB and the MSB of the position value in the frame define
the value range. The technology module can read a position value with a maximum length
of31 bits and communicate it to the controller. The position value is treated as unsigned
positive value and can assume values between 0" and "2
(MSB-LSB+1)-1
".
Complete SSI frame
Instead of having a measured variable returned, you can choose to have the least significant
32 bits of the current unprocessed SSI frame returned. This provides you with encoder-
specific additional bits, such as error bits, in addition to the position value. If the SSI frame is
shorter than 32 bits, the complete SSI frame is returned right-aligned and the top unused bits
are returned with "0" in the feedback interface.