The Modbus protocol includes an internal Modbus ASCII to SPA protocol converter. The SPA protocol is available as a resident extension to the Modbus ASCII protocol and it operates only through the asynchronous serial interface. This interface provides connection to gateway products requiring SPA.
The SPA protocol reuses the settings available for the Modbus ASCII link. The link characteristics are similar in the SPA protocol and Modbus ASCII protocol standards (1 start bit, 7 data bits, even parity, 1 stop bit).
The Modbus unit number setting is reused as the SPA server number. No additional protocol mode parameter exists. The protection relay's Modbus ASCII link detects the incoming client messages and automatically adjusts itself according to the protocol. This switching happens seamlessly, restarting the protection relay is not required.
The SPA conversion from or to Modbus ASCII is done according to the predefined rules. The basic principle is that all data available for the Modbus interface is also available for the SPA protocol. If data is not available through the Modbus interface, it is not available for the SPA protocol either.