turck BL20 Series - 12.5 SSI-interface - BL20-1 SSI; 12.5.1 Data transfer method; 12.5.2 Data exchange

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2020/09
12.5 SSI-interface – BL20-1SSI
The SSI module is used for connecting SSI encoders with a maximum word length of 32 bits and a
maximum bit transmission rate of 1 Mbps. It provides a 24 VDC (500 mA) power supply. In order to
read SSI encoder data, the module outputs a clock signal with which the encoder value can be read
via the signal input. The clock signal and the signal input are based on the RS422 protocol.
12.5.1 Data transfer method
The BL67-1SSI module enables the SSI data to be transferred according to the requirements of the-
application. An operational data transfer method can be set by the module’s parameters.
 Gray code or binary code data transmission is possible.
 Bit transmission rates from 62.5 kbps…1 Mbps are possible.
The SSI encoder value can be represented in a data frame with between 1…32 bits. Bits can be deac-
tivated at both the LSB and MSB side of the frame. At the MSB side this is done by a masking opera-
tion, which causes invalid bits to be set to 0. At the LSB side, the invalid bits are removed by shifting
the entire data frame to the right. The missing bits on the MSB side are filled with zeros.
12.5.2 Data exchange
The data transfer between the PLC to the SSI-module is cyclic. The cyclic data transfer from the PLC
to the SSI-module is done via the Process output data (page 425), The cyclic data transfer from the
SSI-module to the PLC via the Process input data (page 422).
The process output data is used for writing the registers and requesting data from them. It is possi-
ble to stop the communication with the SSI encoder and activate or deactivate comparison opera-
tions.
The process input data is used for reading the contents of the registers inside the modules. In this
case, the SSI encoder value is part of the register. The writing of these registers can be controlled.
The results of different comparison operations can be supplied, and the communication status with
the SSI encoder can also be displayed. Status messages that were generated by the connected SSI
encoder can be passed to the PLC as process input data.
The diagnostics messages are also embedded in the process input data.
The parameter and diagnostics interface allows acyclic data to be transferred in addition to this
cyclic data. The parameters for the data transmission on the SSI module, such as bit transmission
Fig. 289: BL20-1SSI