AS-Interface
AS-Interface (AS-i) is the simplest of the industrial networking protocols used in PLC, DCS and PC-based automation systems. It is designed for connecting simple field I/O devices (e.g. binary (ON/OFF) devices such as actuators and sensors, rotary encoders, analog inputs and outputs, push buttons, valve position sensors ...) in discrete manufacturing and process applications using a single 2-conductor cable.
It is an 'open' technology supported by leading automation vendors. Well over 14 Million AS-i field devices are installed and proven globally.
AS-Interface is a highly efficient networking alternative to the hard wiring of field devices. It is an excellent partner network for fieldbus networks such as *Profibus, *DeviceNet, Interbus and *Industrial Ethernet, for whom it offers a low-cost remote I/O solution. It is proven in hundreds of thousands of applications, including conveyors, process control valves, bottling plants, electrical distribution systems, airport carousels, elevators, bottling lines and food production lines.
AS-i provides the ideal basis for Functional Safety in machinery safety/emergency stop applications. A special profile called Safety as Work (sometimes called ASi-Safe) is used. Safety devices are connected on the same cable as the control system and can provide Safety support up to SIL (Safety Integrity Level) 3 according to IEC 61508 as well as CAT 4 according to EN954-1.
The AS-Interface specification is managed by AS-International, a member funded organization located in Germany. Several international daughter organizations exist around the world.
Components
An AS-Interface network requires only a few basic components. They are:
- Scanner/Gateway (also called master)
- Power supply
- Modules (also called slaves)
- Network cable
Scanner/Gateway
The Scanner/Gateway performs two functions. With respect to the AS-Interface network is a master, performing the data exchange with the modules. Any given network can only have one Scanner/Gateway. With respect to a connected PLC/DCS or PC the Scanner/Gateway is a slave. The AS-Interface community typically uses the word Gateway when the AS-Interface master connects to an upper-level network. On the other hand, if it resides on the backplane of a PLC it is usually referred to as a Scanner. Since AS-Interface communication uses the Master-Slave communication method, any network must have only one Master at a time.
Power supply
Any AS-Interface segment must be powered. This is typically accomplished connecting an AS-Interface power supply. These supplies have certain unique characteristics regarding internal circuitry and output voltage. Standard 24VDC power supplies can not be used to directly power a segment
There are 3 types of cables, yellow jacket, black jacket and red jacket. The first one is the standard cable and have two wires, the brown one is the positive and the the blue one is the negative.
Modules
This is by far the largest group of components and includes binary and analog I/O modules, stack lights, pushbuttons, sensors with integrated ASIC, valve control boxes, E-stops, light curtains; in general any device that can exchange data with the PLC. Each module on the network must have a unique address. For AS-Interface the address space ranges from 0 to 31, where 0 cannot be used, but is reserved for automatic single-node replacement. Since adoption of specification 2.1 this address space is further divided into A and B extended addresses. As a result, using a module designed to support this addressing mode, it possible to have two modules at each address; one at the A half and one at the B half. (Ex. 1A and 1B, 17A and 17B)
The current specification is revision 3.0 and adds many the ability to construct many new types of I/O combinations, including binary modules with 4 inputs and 4 outputs supporting A/B addressing.
Network cable
The vast majority of AS-Interface installations utilize the AS-Interface flat cable, defined as part of the AS-Interface specifications. While the shape of the cable does not matter (any other cable can be used) the electrical characteristics of the selected cable matters greatly. To prevent problems due to improper cable, most professional suggest the AS-Interface flat cable.
