ABSTRACT
AS-i stands for actuator sensor interface and is a bit-oriented master-slave type open fieldbus protocol designed for operation at the device level of a process control system. It is based on modular components and acts as a bridge between the network and binary sensors. Devices can added/deleted in hot plug condition. Data capacity ranges from 1 to 16 bits per device.
It is a cyclic polling, single master, multi-slave type system, the slaves having specific addresses. Topology used can be ring, linear, star, or tree. “Alternating pulse modulation technique” is used during transmission of information—it reduces the bandwidth and also the “end of line reflection” as is very common in square wave pulse technique. Data integrity is protected by sending synchronization information to the receiver. Error detection and retransmission of incorrect data is possible in this scheme.
AS-i protocol conforms to OSI, but layers 3–7 are not used. Above layer 7, there is a layer 8 called “execution control.”
In the initialization phase, parameters of devices are set offline. In the start-up phase, master detects and activates the slaves. In the normal operation phase, cyclic data transfer takes place between master and slave, also acyclic management tasks and inclusion function are executed.
