Frequently Asked Questions
This section provides an overview of AS-Interface
and what it can do for you. It will appeal mostly to 'novices' or those
having little or no knowledge of AS-Interface. More experienced visitors
may wish to read our more technical Forum Page
as well, where many more detailed questions and answers can be found on
different aspects of the AS-Interface protocol.
It is a low-cost electromechanical connection system
designed to operate over a two-wire cable carrying data and power over a
distance of up to 100m. Longer distances can be accommodated if repeaters
and other devices are used (it's
worth reading this for an update). It is especially suitable for lower
levels of plant automation where simple - often binary - field devices such
as switches need to interoperate in a stand-alone local area automation
network controlled by PLC or PC. At its simplest, AS-Interface is a digital
replacement for traditional cable architectures. However, as you'll discover,
it can handle lots more as well and can be an important part of an automation
hierarchy and strategy. A special AS-Interface chip has been developed for
integrating in user modules and field devices, ensuring low cost, robust
and interoperable performance.
AS-Interface should not be compared with higher level
fieldbuses providing sophisticated performance at much higher cost! AS-Interface
is tailored to the needs of devices such as sensors and actuators where
low connection cost per node is critical and simplicity is paramount. Furthermore,
established skills can still be utilised so AS-Interface ensures fast uptake
and ready acceptance by existing operating and maintenance personnel. Many
gateway products are now available, making AS-Interface ideal for interfacing
field I/O with a fieldbus. Ethernet links are also available, enabling an
AS-Interface network to be controlled directly over a TCP/IP network. Thus,
AS-Interface actually makes an ideal partner for fieldbuses operating at
higher levels of automation.
During 2004, market research showed that more than
10, 000, 000 slave AS-i devices were installed globally. This figure is
similar to that of PROFIBUS, the most successful fieldbus in the world.
AS-i therefore is the leading actuator/sensor level networking protocol,
offering proven technology and the support of all the world's leading automation
companies. At time of writing, the toal number of installed nodes is likely
to have passed 11,000,000.
Typically, cost savings are 15-40% compared with
traditional cabling methods, whilst 'per node' connection costs can be in
the range £12-£15, including engineering. However, the ongoing benefits
of digital messaging ensure enhanced operations and maintenance too, which
means whole life costs are much reduced whilst product quality and consistency
are significantly improved because of better performance and less downtime.
Not at all. In fact the simple digital structure
of AS-Interface messages helps rather than hinders the user. Anyone who
understands cable tree topologies will understand AS-Interface with little
additional training. Installing a network is much faster and configuration
takes typically minutes instead of hours since errors can be identified
very quickly. Further, the special yellow cable makes network updates fast
and simple so your AS-Interface network can easily meet the rapidly changing
needs of modern production environments with little additional investment.
AS-Interface developers took great care to create
a simple cabling method that could carry both power and data, AND which
would provide easy connection. Their solution was the distinctive yellow
cable which has become the hallmark of AS-Interface networks. Two features
make it stand out - the special mechanical profile which guarantees correct
connection every time, and the insulation displacement connection technology
(shown right), which ensures fast connection and disconnection. By the way,
conventional round profile cable can also be used in an AS-i system since
many vendors supply screw terminal options and network extender units.
Once again, the developers thought of that. Power
typically comes from a 24V floating DC supply which is fully isolated from
the data signals. Next, data is modulated in a special way that ensures
electromagnetic effects are neutralised, both for radiated and absorbed
energy. Finally, connected devices incorporate software features to eliminate
data 'confusion'. Tests show that an AS-Interface network is so robust,
stable and reliable that it can be run next to high voltage welding systems
with no downgrading.
An AS-Interface network is typically rated up to
8A though the cable itself can handle more and sensible design allows currents
higher than 8A to be drawn provided voltage drops in the network are as
prescribed by the AS-Interface specification.
Not at all. Keeping high power runs short can accommodate
most needs, but if higher currents are demanded over longer distances then
a separate power supply (or combi power supply) and cable are used. This
is possible within the AS-Interface electromechanical spec. by means of
a secondary (black) power cable. The black cable has the same mechanical
profile and is used for supplying actuators, relays etc. A secondary 240V
AC power supply uses a red version of the cable.
Each AS-Interface network needs a 'master', which
can be a local controller box having limited functionality. However, most
masters are built into a separate controller - often a PLC - using either
built-in or plug-in modules. PC interface cards are also available. The
host PLC or PC runs the automation program via the master, which polls the
network issuing commands and receiving and processing replies from connected
devices in the usual way. A gateway to a higher level fieldbus or Ethernet
can also be an AS-Interface master. Vendor software can normally configure,
control and monitor the AS-Interface remotely, in which case the AS-Interface
network is usually seen as remote transparent I/O - a highly cost-effective
and simple approach to field or machine wiring.
Any kind of conventional two-state I/O device can
be connected using a 'User Module' - which is actually an intelligent slave
with a built-in interface chip. Many vendors have also launched stand alone
'intelligent' slaves which use the AS-Interface protocol for more than simple
switching, e.g. parameterising a proximity switch on-the-fly. A comprehensive
range of accessories including tee-pieces and extension units has been developed
by AS-Interface members and a list of available products can be found by
browsing 'Products' in the main menu at the top of this page.
The original AS-Interface specification (V1) allowed
for 31 slaves ('User Modules' count as one slave) to be connected and if
you choose to use only intellligent devices such as a re-rangebale proximity
sensor, then this is the maximum capacity of a network. However, User Modules
can accommodate up to 4 inputs and outputs each. Thus, if 31 User Modules
are connected a total of 124 Inputs and 124 Outputs are feasible, giving
a maximum capacity of 248 I/O per network. A mix of intelligent devices
and User Modules is possible. The recently announced V2.1 Specification
virtually doubles the capacity of a network. For more information about
V2.1 please see later.
Connected slaves are polled in turn by the master.
A fully loaded V1 network offers a maximum response time of 5mS per I/O.
Fewer connected devices means that cycle times are faster. AS-Interface
telegrams (the data messages exchanged by master and slaves) have four useable
output bits, and these are used to control connected devices - e.g. to open
a valve, or close a switch. A slave answers immediately, returning four
bits related to the control function, e.g. confirming the closure of a valve.
The four input and output bits can be used for other functions, in particular
the analog signalling possible with V2.1 devices. Again, please see later
for more information. Some additional data is transmitted during each cycle,
including diagnostics.
Each slave has a unique address which can either
be programmed manually using a simple 'hand-held' or set automatically by
the master. PLC and PC masters also include software that can configure
and control the network remotely as part of the overall strategy.
Version 2.1 Specification enhances the performance
of an AS-Interface network considerably, principally by doubling the number
of slaves to 62. Full backwards compatibility is maintained with earlier
networks and products. Thus a 2.1 master can poll 'old' slaves and new slaves
will operate with an 'old' master, although some new features will not be
accessible in each case. To achieve a doubling in the number of slaves.
A 2.1 master addresses each slave in an A and B mode consecutively using
one of the output bits as a switch. This requires two complete cycles to
address all connected devices so the maximum cycle time of a fully loaded
network increases to 10mS. The number of I/O increases to 186 + 248 = 434.
Other new functions include better diagnostics and a standardised profile
for analog signal handling. By building a 12 bit word across 5 cycles, AS-Interface
can now transmit analog signals for functions such as temperature transmission
and PID control. The Version 2Specification is available free of charge
to members.
This is a further enhancement to the capabilities
of AS-Interface, developed by a Consortium of companies interested in
machine safety and having the objective of introducing
the benefits of networking to safety systems in European markets. A special
slave, called a Safety Monitor, has the single job of monitoring the activity
of Safety Slaves on the network. Typically, a Safety Sensor or Emergency
Stop Button is connected to a Safety Slave and the AS-Interface network
is responsible for transmitting safety signals to the Monitor, which contains
relays to initiate the safety procedures. The Safety Monitor and the Safety
Slaves can co-exist with conventional AS-Interface devices in a normal
network, bringing reduced cabling, easier maintenance and lower system
costs. The profile can meet the requirements of international standards
such as IEC 61508. The AS-Interface Safety at Work profile does NOT change
the responsibilities of system developers and users for administering
their safety strategies correctly, but it can cut costs - particularly
for engineering and wiring - and improve machine operation. Work with
Health and Safety organisations in Europe is defining ways in which networked
safety systems can be properly introduced into Safety markets, to include
training and administration issues and the overriding issue of 'competence'.
By end of 2004 it is believed
that there were over 8000 AS-Interface Safety at Work networks operating
globally. Ongoing developments include extending the Safety at Work capability
to 32 slaves on one network. Read our Safety page
also our Special FAQ on Safety
A consortium led from Japan has developed a simplified
AS-Interface chip for networking panel switches and indicators, to considerably
simplify panel wiring. First products were launched in 2003 and the dominant
company in the sector calls its version 'SwitchNET'. Using the technology
AS-Interface can bring the benefits of networking - lower engineering costs,
less wiring and easier maintenance - to this market with minimal effect
on skills and personnel and at the right cost levels.
Version 3 Specification was announced in late 2004.
It enhances the Version 2 specification by extending functionality to cover
better serial communications,extended IO counts, improved configuration
and parameterisation, synchronised output switching and a number of other
developments aimed at specific market sectors. With Version 3, vendors will
be able to create devices for specialist applications, opening up new opportunities
for AS-i throughout industry. None of the features of Version 2.1 are impacted
negatively and all aspects of Version 3 are fully backwards compatible with
Version 2.1. At time of writing (May 2006) Version 3 devices are beginning
to appear - some of them quite startling in their use of innovation (for
example, read
this). The Version 3 Specification is available free of charge to members.
Yes, you can buy compatible products from over 100
companies so your choice of supplier is not restricted. Interoperability
is guaranteed by conformance testing. International Standards to which AS-Interface
complies include IEC62026 and EN50295. The AS-Interface specification is
available free of charge to all members and no license fees are required
to use AS-Interface devices. The only exception to this is the Safety at
Work profile, which requires anyone integrating the Safety at Work profile
to pay a fee to cover the considerable upfront development costs incurred
by the Safety Consortium. For vendors wishing to manufacture and sell safety
slaves, a small license fee is required, to cover administration costs and
a fixed number of licenses. A list of nearly 300 corporate patents covering
AS-i technology was recently passed over to AS-International to underpin
license-free openness. This list can be scrutinised, although a fee is involved.
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