What is it?
AS-Interface, short for Actuator Sensor Interface, is an industrial networking solution used in PLC, DCS and PC-based automation systems that reliably connect field I/O devices.
What are its basic components?
One network master (i.e. gateway), network slaves (i.e. input and output modules, one power supply and wiring infrastructure.
Where is it used?
Automation applications, including conveyor control, packaging machines, process control valves, electrical distribution systems, airport carousels and elevators.
Who is responsible for it?
AS-International, a member funded organization located in Germany.
Actuator Sensor Interface, commonly referred to as AS-interface or AS-i, is a low-level industrial I/O communication protocol. It was originally intended to be a simple, low cost system that would be easy to install and maintain. With that philosophy in mind, the original developers designed AS-i as a discrete-only two-wire system. It incorporated features such as automatic station addressing, and power and data were carried on a single untwisted pair of wires.
As the demand for AS-i grew, so did the demand for more complex devices. The next major version of AS-i, v2.1, extended the protocol to include seamless transfer of analog data, transmission of simple diagnostic data and an extended addressing scheme that effectively doubled the number of stations allowed on the network. The newest version of AS-i, v3.0, has gone even further, allowing more options for analog data and much more detailed diagnostic information to be communicated. New AS-i masters are backwards compatible with nodes from previous versions.
Through the addition of the Safety at Work (SaW) concept to the AS-i specification, it became one of the first industrial protocols to incorporate safety functionality. It is possible to combine both standard I/O along with safety communication and control on a single AS-i network.
AS-interface is usable as a standalone network, or can be used through a gateway as a subnet to a higher level protocol, such as Ethernet-IP or PROFINET. Gateways are a node to the higher level protocol and a master to the AS-i system.
Typical System Configuration:
Part List: A typical system consists of the following parts:
• ASi Power Supply
• ASi Master
• I/O Nodes
• Junctions and Cabling
The AS-i system uses a freeform topology. AS-i segments up to 100m in total length are possible without any termination. Through the use of a network terminator that length is extended to 200m; active tuners alternatively provide up to 300m total length. Only one terminator or tuner may be used in a segment. Further extension is possible with repeaters. No more than two repeaters may be used in a single direction from the master.
Communication Signal and Power
AS-i communication uses a Manchester II encoded data signal, which results in a very noise immune system. The communication media is a simple two-wire untwisted unshielded cable. Both power and signal are carried over the same pair of conductors. This requires that the DC supply be ‘decoupled’ from the network to maintain signal integrity. Special AS-i power supplies are available which incorporate the supply and decoupling feature in a single package. Alternatively, a separate AS-i decoupler unit or a gateway with internal decoupling can be used, allowing the use of a standard 30 VDC supply.
In many cases the AS-i power supply is insufficient for devices with higher current requirements (particularly output devices). In these cases most manufacturers provide AS-i nodes that draw I/O current from a separate auxiliary supply. The station electronics are generally still powered from the AS-i bus.
The original AS-i system allowed only 4 bits of data to be transferred in each message for a fast and efficient data transfer system. Nodes could be addressed from 1 to 31, but with the growth of the network more than 31 stations were often required. Beginning with AS-i v2.1 stations were available with extended ‘AB’ addressing. This scheme allows the station to be addressed from 1A to 31A or 1B to 31B, allowing 62 total nodes with four discrete inputs and three discrete outputs each. The extended address range (and the limitation to three outputs) is achieved by using one output bit for AB selection.
When both A and B addressed nodes are on the same network, they are scanned on alternating cycles (first all the A nodes are scanned, then all the B nodes). Both AB and single-address nodes can be on the same network. In this case the single-address (non-AB) nodes are scanned every cycle. It is important to note that not all v2.1 nodes use the extended addressing scheme.
Although the original AS-i version only allowed discrete data transfer, v2.1 and higher support seamless analog data transfer. This is accomplished by sending a portion of the analog data on each of several consecutive network cycles; for example, a 16-bit word of data requires seven network cycles. Furthermore, AS-i v3.0 allows analog data transfer in a single cycle by consuming more than one address for the analog node.
AS-interface Safety at Work
AS-interface offers the ability to implement communication and control of Safety data up to SIL 3/Cat. 4 levels. Safety devices can be implemented on the same network with standard I/O. A safety monitor performs the logic for the safety system. These safety monitors are available as an add-on device or as an integrated feature in many gateways. Programming of the monitor is accomplished through the ASIMON software. There are a wide variety of safety input and output nodes available to complete the system.
AS-i has limited field diagnostic capability, due to the limited amount of data transferred in each message. With v2.1, a peripheral fault bit can be reported by a particular node to indicate a fault within the device. This allows the user to easily determine the location of a fault down to the node level. AS-i v3.0 supports expanded diagnostic capabilities; allowing asynchronous ‘mailbox’ messaging for more detailed error information.
Communication Rate/Cycle Time
AS-i communicates at a fixed data rate of 167 kbps. The cycle time of the system is very predictable because of the simple communication scheme and fixed data rate. A network with 31 nodes will have a cycle time of less than 5ms. A fully loaded network with 62 nodes (all A and B addresses used) will have a cycle time of less than 10ms. If analog nodes are being used, the cycle time of those values will increase as a result of the analog values being spanned across multiple network cycles.
TURCK & Bihl+Wiedemann
Bihl+Wiedemann is the leading supplier of AS-i master and gateway products. Their broad product range enables users to select from a wide variety of higher level fieldbuses or PC/PLC control solutions. TURCK has partnered with Bihl+Wiedemann to distribute and support their products in North America. Additionally, both Turck and Bihl+Wiedemann offer a variety of analog and discrete AS-i nodes, PCB devices for OEMs, and sophisticated accessory products.
TURCK offers a complete line of molded AS-i cordsets to facilitate network installation, resulting in a faster start-up and fewer wiring errors. AS-i cables consist of a single untwisted and unshielded wire pair that carries both 30VDC power and the network data. AS-i was originally designed for use with flat cable using an insulation displacement connection, but the use of round cables with sealed connectors has become more common. TURCK provides both cable options.