AS-I (Actuator-Sensor Interface) Basics

AS-I (Actuator Sensor Interface) The Simplest (and Least Expensive?) Fieldbus

Origin: AS-I Consortium, 1993

Maximum Number of Nodes: 31 slaves, 1 master

Connectors: Insulation displacement connectors on flat yellow cable, 2 position terminal block or 12mm ‘micro’ quick-disconnect connectors.

Distance: 100M, 300M with repeaters

Baudrate: 167 Kbits/sec

Message size: 8 bits (4 inputs, 4 outputs) per node per message

Messaging formats: Strobing

Supporting Trade Organization: AS-I Trade Organization, http://www.as-interface.com/

Typical Applications: Commonly found in assembly, packaging and material handling machines. Single-cable wiring of multi-input sensor blocks, smart sensors, pneumatic valves, switches and indicators.

Advantages: Extreme simplicity, low cost, widespread acceptance, high speed, power available on the network. Excellent network for wiring discrete I/O devices.

Disadvantages: Poorly suited to connecting analog I/O; limited network size; limited visibility in North America (at present).

Simplicity In Networking

ASI was developed by a consortium of European automation companies, which saw a need for networking the simplest devices at the lowest level. ASI is easy to configure and low in cost. It is most often used for proximity sensors, photoeyes, limit switches, valves and indicators in applications like packaging machines and material handling systems.

ASI is designed for small systems employing discrete I/O. It allows for up to 31 slaves, which can provide up to 4 inputs and 4 outputs each for a total of 248 I/O.

ASI is arguably the simplest Fieldbus to use. "If you can program a VCR, you can configure an ASI network." ASI uses number of sophisticated and clever mechanisms to ensure fast and reliable data transmission and user friendliness. The only configuration issues are choosing the address of each node and assigning individual inputs and outputs within those nodes.

The Flat Yellow Cable

ASI is best known for its flat yellow cable, which is pierced by insulation displacement connectors so that the expense of tees and complex connectors is avoided. Devices are simply clamped onto the cable and a connection is made. In addition to the popular flat cable, ordinary lamp cord can be used and normally no adverse effects will be experienced.

Power on the Bus

The signal cable also carries 30VDC at low current to power input devices; supplemental power for outputs can be provided via an additional flat (black) cable. Most output devices have provisions for this extra cable.

EMI/RFI Immunity

A common question is the lack of shielding and the obvious concerns about noise immunity. Digital signals are encoded on the cable in a sinusoidal signal, which has a very narrow frequency bandwidth. Filtering which is distributed through the network rejects all extraneous frequencies, and in this way ASI can be operated in electrically noisy environments (even on robotic welders) without experiencing transmission errors.

Analog I/O

Analog signals can be transmitted on ASI, but a node can represent only one analog device, and fragmented messaging must be used to transmit signals requiring more than 4 bits.

Determinism and Scan Time

ASI is deterministic; meaning that one can know with certainty how long it will take for status changes to be reported to the master. To calculate scan time, multiply the number of nodes (including the master) by 150 microseconds. The maximum network delay is 4.7mS, which is certainly speedy enough for most applications (most PLC's have a scan time of 20mS or more!).

ASI and PC Control from Synergetic

Synergetic has a single channel PC card for AS-I which uses the same software and hardware interface as the other cards in our family.  Two Channel ISA and PC104 AS-I masters coming soon, using our universal SyCon Software and Universal Windows Driver and API.
 

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