Analysis and Implementation of a Daisy-Chain Serial Peripheral Interface Bus for a Communication Network with Multiple PLC Modules

In this study, the implications of using the daisy-chain configuration for SPI protocol-based communication between a PLC-CPU and multiple expansion modules have been thoroughly investigated. Equations for clock cycles and data length, for write/read functions have been derived and validated using oscilloscope outputs. Write and read functions have been confirmed by LEDs and the Modbus Poll software, respectively. A comparative analysis has been carried out to highlight the advantages and disadvantages of the daisy-chain configuration compared to the regular SPI configuration. When communicating with two slaves, the daisy-chain configuration required 1.25 times more bytes and time for writing than the regular configuration, while the read function remained the same as the regular configuration. When communicating with single slave, it required 2.5 times more bytes and time for writing and doubled for the reading. Additionally, this configuration halved CS pin usage, which is beneficial for PLC-CPU modules with limited ports.