Technology Modules
D300717 1211 - BL20 I/O modules
10-58
10.4
SSI Interface BL20-1SSI
The BL20-1SSI module is used for connecting SSI encoders with a maximum word length of 32 bits and
a maximum bit transmission rate of 1Mbit/s. 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.
10.4.1
Transmission procedure
The BL20-1SSI module enables the SSI data to be transferred according to the requirements of the
application. The parameters of the module can be configured by the user to set up a functional
transmission procedure as required.
Gray code or binary code data transmission is possible.
Bit transmission rates from 62.5 Kbit/s to 1 Mbit/s are possible.
The SSI encoder value can be represented in a data frame with between 1 to 32 bits. Bits can be
deactivated at both the LSB and MSB side of the frame. At the MSB side this is done by a masking
operation, 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.
10.4.2
Data exchange
The process output data is transmitted from the PLC to the BL20-1SSI module, whilst the process input
data is transferred from the module to the PLC.
The process output data is used for writing the registers and requesting data from them. It is possible
to stop the communication with the SSI encoder and activate or deactivate comparison operations.
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 rate,
telegram length etc. are set via the parameter interface. The diagnostics interface supplies the higher
level system with error messages, such as parameter errors.
10.4.3
Internal registers - read and write operations
The SSI module is provided with a universal register interface that enables access to up to 64 registers.
These are accessed via the process data.
For write access, it must be ensured beforehand that the register write interface is in the default status
and that a write access operation is therefore not currently active. This is ensured if REG_WR = 0 in the
process output data, and is confirmed in the process input data with REG_WR_AKN = 0. Write access is
then possible. The following values must be transferred with the process output data for this:
REG_WR_ADR = Register address,
REG_WR_DATA = Value to be written (32 bit)
REG_WR = 1 (write command)
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