Sigmapi Systems Ltd - AS Interface

AS Interface (ASi)

Fieldbus is a great way of connecting up multiple field devices via a single cable. Savings of up to 40% on initial installation can be achieved by using a fieldbus approach instead of the traditional approach of bringing many hundreds of wires back to a central control unit. Further savings can be made in training and maintenance.

AS-Interface is the lowest level fieldbus system, originally designed to work with analogue devices such as temperature or pressure sensors and proportional valves or variable speed drives. The range of devices that can be connected to an AS-Interface (ASi) network is now extremely large (for example, those offered by Bihl-Wiedemann) making ASi truly flexible.

ASi networks can be easily connected to higher level networks, such as ProfiBus or Ethernet, allowing management level systems to have full access to all information supplied by ASi (including all diagnostics).

When choosing a fieldbus system, you need to take into account:

how easy is it to use?
how flexible is it to install?
does it support modular design?
are there a variety of products available to suit many different applications?
is it easy to set up an installation and replace existing products should they fail?
will it remain compatible with additions to the system made over time?

ASi ticks all these boxes – here’s how.

In order to install an ASi network the first thing you need is the special ASi cable – bright yellow, IP67, flexible, 2-core cable which is run round the plant to allow you to connect your field devices as and where you need them. The cable supports the network and carries both power and data communications.

To run the network, an ASi master is required which communicates with all the field devices (slaves) on the network. If you wish this network to communicate with a higher-level network, an appropriate ASi master gateway is used (for example, an Ethernet gateway). Next is the ASi power supply – a special power supply which ensures that power and data remain separate.

Now, your field devices. This depends on your application, but the devices simply snap onto the yellow cable and are designed to tap into the power and data wires in the yellow cable. If you need to move a device, it can be disconnected from the cable and connected in its new position – the holes left in the original position “heal up” retaining the IP67 rating of the cable. Devices are available in IP20 (if you wish them to be positioned in a cabinet) or IP65 (sometimes, IP67) if they are to be exposed to the environment.

Some slaves require more power than can be supplied by the yellow cable but there is a supplementary black cable which can supply auxiliary power.

Each slave has LEDs showing the health of the device (very useful for quickly spotting faulty slaves) and should one go faulty, it can be “hot swapped” (replaced without affecting the operation of other devices on the ASi network and without requiring any restart).

ASi supports modular design in that a network can be as simple as a straight run of ASi cable with slaves attached, it can have branches leading off the main “back bone”, it can even link to other ASi networks. There are limits to the length of cable, but there are various methods for extending that whilst retaining full communications and low levels of noise.

ASi networks can be used in any industry for production line automation, test rig control and monitoring, electrical distribution systems and many others.

Furthermore, using the ASi Safe (Safety at Work) profile, machine safety can be implemented on the same two-wire cable as the ASi control and monitoring. Designed correctly, safety performance up to SIL3 can be achieved. This can be added to existing ASi networks or designed in as part of a new installation.

By adding an ASi safety module and safety slaves (such as emergency stops, machine interlocks, light curtains), plant can be run safely. The protocol between the safety module and the safety slaves ensures that, in and emergency, all or part of the plant can be shut down immediately. With good design, safety zones can be implemented easily.

In addition, because the safety module is seen as any other slave on the network, full diagnostics can be supplied to any management level system connected to the network.

In summary, the main benefits from using ASi include:

major reduction in cabling
power and data over the same cable (although a dedicated power cable can be used as well for applications requiring the extra power)
highly reliable communications (high levels of noise immunity) on low-cost 2-core cable
faulty devices can be replaced simply
cable is IP67 and many devices are also available to IP67
safety system can be fully integrated.

Sigmapi have Certified ASi Engineers who will be happy to discuss your requirements with you – from consultancy to full designs. We also add value to the benefits of ASi by:

Integrating ASi with other technologies
Providing you with an easy to use operator interface providing you with full diagnostics
Designing and installing PC or PLC based systems (or a combination of the two) incorporating zoned safety (should you require it).

Best viewed with Mozilla Firefox Last updated 27/08/10