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FieldLink™ – Process networking AS-Interface

FieldLink™ – Process networking AS-Interface

Stonel FieldLink program enables you to take advantage of field-based communication technologies to cut installation and maintenance costs and improve operating performance. In thousands of applications Stonel has demonstrated installation savings up to 40%, improved flexibility and reduced infrastructure requirements. With communication networks, vital predictive maintenance and performance measures may also be transmitted from field instruments to the asset management or operation control centers further reducing maintenance costs. A broad range of networking protocols is supported. They include AS-Interface, DeviceNet, Modbus, Profibus, Foundation Fieldbus and more.

AS-Interface networks up to 62 field devices onto a single pair of wires that delivers both signal and power. Actuator Sensor Interface, or AS-Interface, was developed by a group of sensor manufacturers and introduced into the market in 1994. Since that time, it has become the de facto standard for discrete sensors in process industries throughout the world. AS-Interface may be used in hazardous process environments using explosion proof, tray cable, nonincendive and for nonincendive wiring protection concepts.

Features

• Ideally suited for on/off batch process valves and other discrete applications.

• 62 field devices per network master.

• Simple electronics for economical and robust performance.

• Transfer medium is unshielded two-wire cable for both data and power supply.

• Signal transmission has high tolerance to EMI.

• Easy to install providing the greatest cost savings with the least complexity.

• Free choice of network topology allows optimized wiring network.

• Variety of gateways available to seamlessly tie into high level bus networks.

Overview and Analysis

AS-Interface is reliable

The AS-Interface Alternating Pulse Modulation (APM) with Manchester II coding and decoding minimizes electromagnetic emissions and is highly tolerant of electromagnetic interference. AS-Interface has a very high level of data integrity and is classified as I3 according to DIN 19244. These networks are robust and perform reliably under the most strenuous environments. AS-Interface networks are recommended for “mission critical” applications.

AS-Interface V.S. Conventional system

AS-Interface is a versatile, low cost alternative to traditional hard wired I/O. It can replace traditional point-to-point wiring with a better, more flexible solution that is easier to install, operate and maintain and easier to re-configure.

Typical batching valve wiring networks attach each of the inputs and outputs (I/O) to a central location resulting in multiple wire runs for each field device. See figure 1. Large expenditures are needed for cabling conduit, installation and I/O points. Space for I/O racks and cabling must be accommodated in order to attach only a few field devices.

Figure 1

A simple gateway interfaces the network into the field communication bus. See figure 2. Data and power are transferred overthe two-wire network to each of the AS-Interface compatible field devices. Each valve communication module contains an AS-Interface ASIC and other electronics to gather open or closed position status and power solenoid or other ancillary devices on or off. Other AS-Interface modules are available to gather inputs and switch power outputs.

Figure 2

AS-Interface economic analysis

Using a network with 16 valve communication terminals (VCTs), the following economic analysis may be performed (costs are listed in the amount per automated valve):

There is a net savings of $1,335 per automated valve with AS-Interface communication over the conventional system (52% reduction in installed cost). This savings does not include other cost reductions due to less space consumption for wiring, conduit and I/O racks, as well as greater flexibility in adding field devices or reconfiguring later.

Calculated per field deviceConvantionalAS-Interface

Calculated per field device Conventional AS-Interface
Computer I/O; Master/gateway  $ 70,00  $ 65,00
Conduit, cable tray, wiring and fittings  $ 1 500,00  $ 350,00
Valve monitor/VCT and pneumatic valve  $ 510,00  $ 610,00
Switched protected drop connector  n/a  $ 90,00
Installation and commissioning labor  $ 700,00  $ 300,00
Power supply  $ 50,00  $ 30,00
Total installed cost  $ 2 780,00  $ 1 445,00
Total installation savings
$1,335 per automated valve
Technical information

Power and data

In an AS-Interface network, data and power are carried over a singletwo-wire cable that links up to 62 field devices. Each of the fielddevices may have up to 4 inputs and 4 outputs for a total of 248 binary inputs and outputs per string. Analog inputs and outputs are also available.

Transmission media

Two-wire unshielded, untwisted cable, 2 x 1.5 mm2

(16 AWG) is recommended for data and power in the process environment. 100 meters (328 feet) total length of cabling may be used for each master/gateway. Additional length of 100 meters for each repeater with up to two repeaters in series. Parallel repeaters can be used to construct long networks, provided there are no more than two repeater hops from each device to the master. Total number of field devices must remain at 62 regardless of number of repeaters for each master.

Data integrity

When classifying the bus in terms of data integrity (according to EN608 70-5-4-DIN), the AS-Interface network falls into the highest data integrity class even with a bit error rate of 10-3. Such accuracy is suitable for the transmission of mission critical information.

Topologies

AS-Interface is capable of supporting any topology. Any combinations of star, ring, tree and linear are possible. Use of a repeater in a ring is not permitted.

Standards and user groups

AS-Interface technology has been standardized in EN 50295 and IEC 62026-2. There are over 400 different products available from 50+ vendors. And, as of this writing, there are estimated to be over 12,000,000 installed nodes (field devices) throughout the world.

AS-Interface system specifications

Topology Linear, star, tree or ring
Adressing AS-I master or handheld
Signal coding Manchester type with alternating pulse modulation
Cycle time 10ms maximum with 62 devices
Transmission rate 167 Kbits/s
Data per message 4 bits bi-directional
Access procedure Master/Slave
Error detection 1 parity bit + signal quality monitoring
Number of field devices 62 per network
Cabling Unshielded 2-wire
Cable length 100 m per master or 300 m with 2 repeaters