Everything you need to know about AS-Interface
So many protocols! How do you pick the right one for your application? Many offer nearly identical solutions. Today, let’s dive into the AS-Interface, also known as AS-i, to figure out how to apply it.
The market needed a basic protocol fast enough to connect field sensors and actuators to their controllers. AS stands for “actuator-sensor,” and in 1990, a consortium of companies in Germany developed this interface.
In 1991 they founded the AS-International Association, also known as the AS-Interface UK Expert Alliance. Members of this alliance, end-users and vendors alike, applied the technology and aspired to make it an international standard. The alliance also established a good relationship with another association, PROFIBUS International. In 1999, the AS-Interface won its standardization, designated EN50295/IEC 62026-2.
You can learn more about AS-i from the AS-International Association official site. You can also find an app that guides users in proper installation.
Some characteristics of this protocol:
- Low cost in comparison to similar protocols
- Reliability in many conditions
- Flexible network
- Easy expansion if necessary
- Simple installation
How does it work?
AS-i gives you a standard master-slave network with cyclic polling. Moreover, as an open and independent bus system, it guarantees a seamless integration.
All sensors and actuators connect to the control system using unshielded two-wire lines that provide both data transmission and energy. This system has a structure that breaks down into three levels.
Interface 01: Sensors and actuators
At this level, we have the slaves using AS-i chips. This chip allows the slaves to connect to the network, send data to the master, and receive commands back. An analog device in the network that sends data larger than 4 bits will divide it into many cycles to send to the master.
Interface 02: Transmission system
At this level, the network exchanges data among the slaves in the network. Interface 02 defines how to access Interface 01, exchange electrical data, and handle communication issues. It also defines the time required in each transaction.
Interface 03: Control element (master)
At this level, the master connects the controller (host) to the sensors and actuators. Furthermore, the master can manage traffic independently and set up and run diagnostics.
How do you connect sensors and actuators to an AS-i network?
The connection of the field sensors follows the EN50295 standard. You can use a handful of topologies here, such as tree, star, line, or combinations of each.
AS-i technology serves as all the structure needed for communication. Here you have a single special cable developed for AS-i networks.
Along with the signal, you have a power supply of 24 volts of direct current (VDC), and you can connect up to 31 slaves to the network using the traditional system. This setup has a response time of five milliseconds (ms). The new version allows up to 62 slaves with a response time of 10 ms.
Developers created a yellow flat cable explicitly for this network, but you can also use a round cable for AS-interface. And you may sometimes see a black flat cable used to supply extra voltage for actuators that need more juice.
Interestingly enough, the yellow cable doesn’t need cutting or peeling to set devices on it. Basically, you penetrate the insulation using conductive blades to connect it to the internal copper wires, and the cable self-heals after you remove the slave. However, nowadays the round cable with m12 connectors has become popular to connect slaves to the trunk line.
Components of AS-Interface
Like any other network, you have essential elements needed to build an AS-interface system. Let’s see the possibilities and how these components act!
As you already know, the master sends the signal from the sensors to the controller and keeps the cyclic polling. Depending on your network structure, you can have gateways or even a programmable logic controller (PLC) working as the master. Let’s review these options.
- Gateways: These devices work as masters on the AS-i network, controlling the slaves and sending data to the controllers. They can connect an AS-i network to other networks such as DeviceNet, Profibus DP, Modbus, and Ethernet.
- Stand-alone controllers: These control the network and run control programs on their own. You can also connect them to a PC to set up and collect data, but they don’t need a PC to work.
- PLC cards: These cards, installed on the PLC backplane, allow you to reduce your number of wires. You can find many vendors offering AS-interface cards for their PLCs.
- PC cards: These work like the PLC cards, only for standard computers.
The power supply provides between 29.5 and 31.6 VDC to the slaves and master, and it provides current from zero to eight amps. On an extended network, you need to remember you’ll get a voltage drop in the network that you must keep below three volts. Furthermore, the power supply balances the network using an internal balance system, the only point of the network connected to the ground.
As already mentioned, you have cables explicitly designed for the AS-interface, the yellow flat wire for the slaves, the black cable to provide extra power if needed, and the round cable with its M12 connector.
Depending on the network topology, you can use an active or passive junction to connect the slaves to the trunk line.
AS-Interface network topology
Now let’s take a look at how to connect the lines to the nodes. Usually, we use the term topology for buses. The maximum cable length is 100 meters.
- Line – Simple installation where the slaves connect one to the next.
- Branch – Here the trunk line has spurs or drops to connect the slaves.
- Star – This layout has the trunk line connected to an active and passive junction, and then all the buses radiate from there.
- Tree – This topology creates good flexibility, with the main trunk and multiple branches.
While the network cable has a maximum length of 100 meters, you can expand the network using extenders or repeaters.
AS-Interface versions and specifications
Since its development, we’ve seen several versions enhancing the network. If you want to see them all, then you can visit the AS-i site, but here is a small sample:
- Original specification (1994, version 2.4)
- 31 slaves
- 124 inputs/124 outputs
- update time of 5 ms
- Enhancement (1988, version 2.11)
- 62 slaves
- 248 inputs/186 outputs
- 10 ms update
- Additional capabilities (2005/2007, version 3.0)
- Binary I/O nodes to support A/B addressing with 4/4 inputs and outputs
- Binary I/O nodes holding A/B addressing with 8/8 inputs and outputs
- Fast analog channel with a configurable (8, 12 or 16 bit)
- Full duplex bit serial data channel