AS-Interface
AS-Interface (Actuator Sensor 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.
AS-Interface 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 higher level 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-Interface provides the ideal basis for Functional Safety in machinery safety/emergency stop applications. Safety devices communicating over AS-Interface follow all the normal data rules. The required level of data verification is provided by dynamic changes in the data. This technology is called Safety as Work (or ASi-Safe) and allows safety devices and standard to be connected to the same network cable. Using appropriate safe input hardware (i.e. light curtains, e-stop button, door interlock switches ...) AS-Interface 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.
History
AS-Interface was developed during the late 1980 and early 1990 by group (consortium) of 11 companies mostly known for their offering of industrial non contact sensing devices like inductive sensors, photoelectric sensors, capacitive sensors and ultrasonic sensors. Once development was completed the consortium was resolved and a member organization, AS-International, was founded. The first operational system was shown at the 1994 Hannover fare.
Original Specification (1994, Version 2.04)
In its original form the network was capable of supporting up to 31 binary I/O devices, where each device could exchange 4 bit of input and 4 bit of output data, resulting in a total of 124 inputs and 124 outputs on a single network. Important features like Automatic Single Node Replacement where already part of the system. The network update time is easily calculated by multiplying the number I/O nodes with the deterministic update time for each node (approximately 150 microseconds). This simplified calculation does not include the Management Phase which is negligible for typical installations.
Enhancements (1998, Version 2.11)
Following its introduction users quickly adopted AS-Interface, driving the demand for additional functionalities and features. As a consequence. These demands were addresses with certain specification enhancements allowing the creation of analog input/output devices and increasing the number of possible binary I/O devices to 62. Diagnostics functionality was also enhanced by the creatin of the Peripheral Fault Bit. In oder to retain full forward and backward compatibility the size of the data frame exchanged between Scanners and Gateways was not increased. Instead, one of the 4 output bits was used to select between teh so-called A and B nodes. This enabled each of the 31 addresses to be used twice. As a consequence the fourth output bit was not available to the user and binary I/O nodes build to this new profile offered a maximum of 4 inputs and 3 outputs, increasing the total amount of I/O on a single network to 248 inputs and 186 outputs.
Additional capabilities (2005/2007, Version 3.0)
By 2005 the worldwide success of AS-Interface, approximately 10 million nodes in operation, made it necessary to address additional user requirements. Also, the increased usage of Ethernet based industrial protocols, called for a low level solution that overcome the inherent shortcomings of Ethernet (e.g. restricted topology, large data frame, costly usage of switches ...) This specification addressed the users requirements by defining new communication profiles for binary and analog data plus the introduction of a serial data transmission profile. The following is an incomplete list of the new capabilities
- Binary I/O nodes supporting A/B addressing with 4 Inputs and 4 Outputs
- Binary I/O nodes supporting A/B addressing with 8 Inputs and 8 Outputs
- Configurable (8, 12 or 16 bit) fast analog channel
- Full Duplex bit serial data channel
With these new capabilities, AS-Interface becomes the ideal partner network for any of the currently available Ethernet based industrial protocols. Gateways to EtherNet/IPTM, PROFINET, Modbus/TCP and others are available. Some controls experts have voice the opinion that within the next 10 years networking solutions positioned between AS-Interface and Ethernet will not be used in any new installation.
Components
An AS-Interface network requires only a few basic components falling into the following general categories:
- Scanners and Gateways (also called masters)
- Power supplies and repeaters
- Modules (also called slaves)
- Network cable, installation hardware and useful tool (infrastructure)
Scanners and Gateways
The Scanner/Gateway performs two functions. With respect to the AS-Interface network it is a master, performing the data exchange with the modules and updating its internal I/O image. The functionality of the master is defined in the Master Profile of the AS-Interface specification. As part of specification version 3.0 the M4 Master Profile has been defined. 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 like DeviceNet, Profibus or any of the industrial Ethernet flavors. 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 is based on 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. The total length AS-Interface network cable in a single segment must be no more than 100m. If the total network length must be longer repeaters can be used. As the repeater galvanically isolates any two segment a new power supply must be used on the far side of the repeater. A common misconception exists concerning the number of repeaters in a network. It has been stated that the maximum length of an AS-Interface network can be 300m, created by using two repeaters. This is not the case at all! What matters is not how many repeaters are using but rather how many repeaters any data packet originating at a Scanner or Gateway has to cross before reaching the I/O node. Due to the tight timing constraints defined each packet can at most travel across two repeaters before reaching an AS-Interface node. This has the following consequences:
- Linear networks with the Scanner/Gateway mounted at one end can be 300m long
- Linear network with 500m length can be constructed when the Scanner/Gateway is mounted in the middle segment
- Star shaped networks with virtually no length limitation are possible
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 2.11 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 Version 3.0 has 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. This cable is designed to make use of the cable piercing technology. When an AS-Interface module is installed on the network, piercing needles penetrate the cable and displace the internal copper strands without cutting them. This allow AS-Interface modules to be installed anywhere on the network without cutting and preparing (i.e. removing cable jacket, stripping insulation and possibly applying a ferule) the cable first. The result is a faster installation without the chance of inadvertent shorts between the leads.
There are several types of cables available. Yellow cable is usually used to power AS-Interface modules and enable communication between the field devices and the scanner or Gateway. Several material are offered to address specific applications needs. The AS-Interface black cable is typically used to supply modules with 24VDC AUX power. No communication takes place of this cable. Similar to the yellow cable, the black cable is also produced using various jacket material to address the specific needs of the application. A red jacketed cable has been defined but is virtually unused. Its intended use was in applications where AC power is supplied to the field nodes. The two leads inside the AS-Interface cable are brown (+ lead) and blue (- lead) independent of material makeup and outer jacket color.
Other components
Passive taps, flat-to-round cable adapters, handheld addressing tools and many other accessories are designed to further simplify the installation of AS-Interface networks.