AS-Interface (actuator sensor interface) is a networking solution intended for the industry and used in PLC-based, DCS-based and PC-based automation systems. The AS-Interface was developed to work for the connection with simple ON/OFF devices. This includes for example actuators, sensors, encoders, and push buttons in the manufacturing industry.
Currently, the AS-Interface is being used on over 18 million devices. This points to a growth rate of approximately 2 million users worldwide. This high number of users has been reached because the technology is freely accessible to all device manufacturers and developers.
Examples of use
AS-Interface is a real alternative to conventional network solutions. For example, it can be used as a partner network for higher-level systems such as Profibus, DeviceNet or Interbus. The AS-Interface already is being used for automation processes, packaging machines, process control valves, filling installations, electrical distribution systems, elevators and in the field of food production.
AS-Interface also offers advantages from the point of view of security. Linkages with safety hardware such as emergency off switches, light curtains and door lock systems can provide a safety integrity level (SIL) of 3. SIL 3 is comparable with other standards such as EN 62061, CAT 4, EN954-1, performance level e and EN ISO 13849-1.
The AS-Interface was already developed between 1980 and 1990 in cooperation with 11 companies that merged to become AS-international. The first functioning system was presented a short time later at the 1994 Hannover Messe.
In its original form (1994, version 2.04), the network was able to exchange up to 31 binary I/O devices, in each case with 4 bit input and output data. This corresponds to 124 inputs and outputs. Important functions such as automatic single node replacement were already a part of the system.
The network updating time is calculated by multiplying of the number of I/O nodes with the deterministic updating time for each node (approximately 150 micro seconds) with a max. change of 5 ms.
An user manual made it possible for users to get along quickly with the new AS-Interface.
The demand for new, additional functions and features grew. Because of this, AS-International was able to increase the number of possible inputs and outputs from 31 to 62 as well as add a diagnosis function based on a peripheral fault bit unit (1998, version 2.11).
In order to reach full upward and downward compatibility, the size of the data frame between the network master (scanner and gateways) was exchanged, but not increased. Instead, one of the four output bits was used to select between the so-called A and B-nodes. This made it possible for each of the 31 addresses to be used twice. The address space was increased to 1A – 31A and 1B – 31B. Because of the use of one output bit as a A/B selector, the fourth output bit was not available for the user and was constructed as a binary I/O node. The profile offered four inputs and three outputs (max.) which permitted an increase in the total I/O quantity on a single network to 248 inputs and 186 outputs. The max. updating time of a fully loaded network is around 10 ms.
Up until 2005, it was necessary to meet additional user requirements. The increasing usage rate of Ethernet-based protocols in industry required made it necessary to find a solution that overcame the more serious defects. Users demanded new communication profiles for binary and analog data, as well as the introduction of serial data transmission. The following is an incomplete list of the new functions:
- 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) quick analog channels
- Full duplex bit serial data channel
With these new functions, the AS-Interface becomes an ideal partner for one of the currently available Ethernet-based industrial protocols and a gateway to EtherNet /IPTM, PROFINET, Modbus /TCP, SERCOS III (as well as others). Some control element experts are of the opinion that within the next 10 years, network solutions will be positioned between AS-Interface and Ethernet, which will not be used with each new installation. In a worst-case scenario, with 62 nodes (with 4 inputs and 4 outputs) the update will be 10 ms for the inputs and 20 ms for the outputs.
An AS-Interface network only requires a few basic components:
- Scanner and gateways (also as master)
- Power supplies and repeaters
- Modules (also as slaves)
- Network cable
- Installation material
Scanner and gateways
The scanner/ the gateway fulfills two functions. With reference to the AS-interface net, the data exchange with the modules and the updating of the internal I/O are executed by a master. The functionality of the master is defined in the master profile of the interface specification. The M4 master profile has been defined as a part of specification version 3.0. Each given network can administer only one scanner/ gateway With regard to a connected SPS/DCS or a PC, the scanner/gateway would be a slave. As a rule, the AS -Interface community uses the word gateways when the AS-Interface master creates a connection to a higher network level such as a DeviceNet, Profibus, or an industrial Ethernet. However, when it is located on the rear wall circuit board of an SPS it is normally called a scanner. As the AS- interface communication is based on the master- slave communication process, each network must have only one master.
Each AS-Interface segment must be supplied with power. Typically, this is ensured by connecting an AS-Interface power supply. A frequent misunderstanding concerns the number of repeaters in a network. Because of the close time-defined constraints, in most cases each packet can be transmitted via two repeaters before reaching an AS-Interface node. Facts:
- At the end, the max. length of linear networks with a scanner/gateway is 300 m.
- The max. length of linear networks with a scanner/gateway in the central segment is 600 m.
- Star-shaped networks do not have a length limit.