AS-i In Depth

By Peter Cheetham, Sigmapi Systems Ltd

AsiActuator

The system works by having one AS-i master unit communicating along a single two-core cable (what has become the trademark AS-i yellow cable2) with the various AS-i actuators and sensors which are simply clipped onto the cable using an insulation displacement technique. The cable is profiled so that it is difficult to fit these devices, called AS-i slaves, the wrong way round. Amazingly, the cable is self-sealing, so that even if a device is removed from the cable, it remains protected to IP67.

In fact an AS-i actuator or sensor is a normal actuator or sensor fitted with an ASi communication chip. This chip has become so cheap it can be incorporated into a something as simple as a panel indicator, providing not just ease of use but also diagnostics (such as lamp failure) back to a control and monitoring system. A separate range of products known collectively as AS-i Safe provide the infrastructure and devices needed to implement fully zoned safety systems which can form part of a basic AS-i system.

Of critical importance is that AS-i is an open technology, not tied to any particular vendor, so companies producing all kinds of specialist devices are now making an AS-i version of their device, which, because of the tightly specified standard is guaranteed to work on your AS-i system. Moreover, if your favourite device is not available in an AS-i format then generic AS-i slaves are available which can be used to connect traditional actuators and sensors onto the AS-i bus.

Perhaps even more importantly, to develop bespoke systems which have to integrate with a client’s existing system, the need to do as much work off site as possible before delivery is paramount. Traditionally this has caused problems because a client’s existing system may be complex or costly to simulate off-site. By using dummy AS-i slaves, the functionality of a new system can be fully demonstrated to the client without using real devices and then implemented on-site with confidence and with extremely short plant down times.

From the system integration point of view, the fact that gateways are also available to connect AS-i onto other fieldbuses and Ethernet means that there is no limit to the practical expandability of this type of system. It also allows integration into management information systems and remote or wireless connectivity.