Esbus - A Sensor Bus Based on the SPI Serial Interface

A local sensor bus for transmitting data from several digital smart sensors to an Internet interface was tested. The master/slave type bus is based on the three-wire SPI (serial peripheral interface) bus, which is built into most micro controllers. At the sensor end, after a/d conversion of the analog sensor signals and further processing of the digitized data, the data is formatted into standard 8-byte ASCII code, which can be read directly by any computer. A header indicates the type of sensor data, for example, TC123.45 for a temperature sensor reading 123.45 °C. The data is in the form of an email so the following Internet interface (TCP/IP) only retransmits the data to the requesting browser where it is displayed in the same format as any email The Esbus format consists of an 8-byte request from the master followed by 32- byte response from the selected slave. The response is subdivided into an 8-byte header, which identifies the sensor and channel number, and three sets of 8-byte sensor data, as described above. In order to adapt the SPI an addressable network, the data output driver is converted to open collector. This allows the device to be multiplexed since logic 0 is equivalent to no data transmitted or slave disabled. Optical isolators allows ground isolation. The paper will describe the i/o circuits, waveform/timing, and data formats for a specific smart sensor (temperature, humidity, illumination).