## Sensor Bus Standards
### Serial Peripheral Interface
#### Transducer-Independent Interface  
The recently adopted IEEE P1451.2 standard introduced the Transducer-Independent Interface (TII) [4-6]. This standard was defined to provide an industry-standard interface to connect sensors to  
Figure 2: Typical SPI bus configuration.  
microprocessors. The objective of the standard is to provide a common digital interface between transducers and the network communications nodes, and consequently facilitate the transducer manufacturers to develop smart devices and to interface those devices to networks, systems, and instruments by incorporating existing and emerging sensor and networking technologies. The physical representation of IEEE P1451 standard is shown in Figure 3.  
IEEE P1451.2 defines the interface (the hardware and software blocks) for a networked transducer that does not depend on a specific control network. It introduces the concept of NCAP (Network Capable Application Processor) and STIM (Smart Transducer Interface Module). It defines a TEDS (Transducer Electronic Data Sheet) as an integral part of the STIM, and it defines the TII bus. An objective of IEEE P1451.2 has been to separate the network issues from the transducer issues. The STIM handles the sensor and actuator low-level interface activities. The NCAP handles the network interface and has a dedicated interface port to the STIM. The TEDS contains detailed information about the transducer attached to the STIM, such as calibration coefficients, type of sensor, model number, data units, etc. TII is the port through which the STIM and the NCAP communicate to each other.  
IEEE P1451.2 defines sixteen functional addresses. They control the STIM as well as reading and writing both data and memories in the STIM via TII. Certain control and status functions are loosely defined and provisions are available to add vendor unique functions.  
TII is a 10-wire digital bus. It is built around synchronous serial communication based on the SPI protocol. Several dedicated lines have been added to provide power, ground and special-purpose control lines, including _NIOE_, _NTRIG_, _NINT_, _NSDET_ and _NACK_. _NIOE_ is used to indicate the access by the NCAP. _NACK_ toggles after each byte to signal that STIM is ready to continue the data transport frame. _NTRIG_ is an NCAP-generated trigger signal which goes active when the NCAP wants the STIM to take action. The trigger can cause a sensor to take new data or apply previous written data to an output. _NINT_ is the service request or interrupt line. The STIM may use this line to generate an interrupt for a processor. Another feature of the TII bus is that it supports a plug-n-play function. When a STIM is connected to the NCAP, signal line _NSDET_ will go low and the NCAP can detect the condition of the pin and detect the installation or removal of a STIM.  
Figure 3: Physical representation of IEEE 1451.2.  
TII is an interface that the sensor manufacturers can use which is independent of the network being used to interconnect the sensors. It provides trigger, interrupt, and plug-n-play functions through dedicated signal lines. The data transport is the same as SPI with 8-bit data words. The software and hardware of the interface are simple, and it can operate with very low power. However, TII allows only one interface module (STIM) to be connected to the microcontroller. It does not support distributed multi-drop sensor modules which are necessary in many sensor network applications. Furthermore, TII needs ten lines between the microcontroller and the sensor module and therefore utilizes considerable controller I/O resources as well as presenting a potential wiring problem.