Actuator-Sensor Interface, commonly known as AS-i Bus, is a standard device-level protocol used in industrial automation to connect actuators and sensors. Hence, the name Actuator-Sensor Interface! AS-i Bus is defined by the standard IEC 62026-2.
But just like we use different versions of our smartphone operating systems over time, AS-i Bus has also evolved, and various versions of the protocol are in use. As an automation engineer, you need to understand the properties of the different versions that are available.
Watch the video below to learn more about this topic in a clear and easy way.
Here is a quick summary of the two versions of AS-i Bus still in use. Values in parentheses are for the enhanced version of AS-i v3.
What happened to AS-i Version 4? Well, there was no Version 4 released. IEC felt the new version was so far advanced compared to Version 3 that the new Version should skip a number. From the chart, it appears that decision was justified!
Compatibility with currently installed versions
Upgrades from AS-i v3 to AS-i v5 are straightforward. In general, AS-i v3 devices are compatible with AS-i v5 networks. However, devices that were designed for AS-i v3 may not support the newer features of AS-i v5, but will still be able to operate on AS-i 5 networks.
Engineers need to confirm that the AS-i Bus version selected supports the necessary communication protocols of their devices and control system.
So, what do I need to consider when implementing AS-i Bus systems? First, I will look at the heart of any AS-i Bus network: the AS-i Master.
The AS-i Bus master card in any control system has many functions. The master controls communication across the network, including polling of each node for data, sharing diagnostics information, and coordinating message timing.
Different AS-i Bus master cards allow the network to have specific properties. For example, the Siemens CM 1243-2 for S7-1200 control systems is an AS-i v3 network master with the properties of the extended AS-i v3 configuration given previously.
By contrast, the Bihl+Wiedemann BWU3852 is an AS-i v5 compatible device which can also be configured to provide an AS-i v3 network.
AS-i Bus data profiles
Another important characteristic is the ability of an AS-i Bus master to provide data profiles compatible with the devices connected to the network.
The IEC 62026-2 standard for AS-i v3 defines data profiles that a device can have, and the engineer needs to ensure that the AS-i master and devices are compatible. For example, here are a few of the profiles supported by the Siemens CM 1243-2.
There are many other data profiles that are defined in IEC 62026-2, including those that support ASiSAFE for high-availability safety applications and other advanced features.
Here is an example from a data sheet for a GEMÜ 4242 Combi-Switchbox used to detect valve position.
Note the various data profiles available for this device. Which one is right for your system? The specifying engineer needs to know the properties of the plant’s system in order to make the right selection.
If the AS-i Bus system installed is AS-i v3, then order codes A5 and A5D are not appropriate.
If the AS-i Bus is version 3 without enhanced addressing capability, then order codes A3 and A4 are not appropriate.
If the AS-i Bus is version 5, order code A5 provides 96 nodes with Bluetooth communication to the device enabled.
If the AS-i master allows for expanded node data, then code A4 could be used to allow for expanded diagnostics.
AS-i Master
Many AS-i Masters have the capability to support two different networks or segments. These are typically referred to as A and B.
This allows for the two segments to have their own set of node numbers, but there cannot be two nodes on the same segment with the same node number.
Again, the engineer must understand the capabilities of the AS-i master in use. Generally, this dual-segment feature is available with many version 3 and version 5 masters.
Each segment has its own configuration setup, which is managed by the control system interface and is limited by the capabilities of the AS-i master hardware.
AS-i Master modules and individual nodes are configured within the control system software environment to which they are connected, or with special software provided with the AS-i master. The bulk of the configuration choices are related to the nodes connected to the AS-i network.
A typical configuration page for an AS-i node will allow me to configure items such as the type of I/O, digital or analog, to be expected from the device node, the name and address of the node, the data profile to be used, and any options that may exist for the node.
Here is an example of an AS-i configuration for a variable frequency drive in a Siemens S7 system.
AS-i Devices
The AS-i Master or an AS-i address programming device is used to perform AS-i addressing on each device. This address is used to identify individual nodes in the AS-i network, and as mentioned previously, addresses cannot be repeated on a single segment.
The AS-i master version determines the range of addresses possible for segments connected to the AS-i master. AS-i addressing is critical because if the addresses of the two AS-i nodes are swapped, the IO will not be assigned appropriately in the control systems data table.
Summary
AS-i Bus versions 3 and 5 provide a simple, robust, and low-cost 2-wire communication bus for controlling devices such as valves, motors, variable frequency drives, proximity switches, and other digital and analog devices.
AS-i version 3 is widely used in industry, but AS-i 5 offers higher communication speed, extended data interface capabilities, and advanced diagnostics.
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