turck articles

AS-interface System Description

TURCK 6/11/2018
TURCK AS-interface System Description

AS-interface (Commonly Referred to as AS-i) is a low-level I/O interface system.  It was originally intended to be a simple, low cost system that would be easy to install and maintain.  With that philosophy in mind, the original developers designed AS-i as a discrete-only two-wire system.  It incorporated features like automatic station addressing, and power and data were carried on a single untwisted pair of wires.

As the demand for AS-i grew, so did the demand for more complex devices.  The next major version of AS-i, v2.1, extended the protocol to include seamless transfer of analog data, transmission of simple diagnostic data and an extended addressing scheme that effectively doubled the number of stations allowed on the network.  The newest version of AS-i, v3.0, has gone even further, allowing more options for analog data and much more detailed diagnostic information to be communicated. New versions of AS-i are backward compatible and support slaves from earlier versions.  Additionally, AS-i was one of the
first network systems to incorporate a safety protocol, allowing emergency-stop and machine-stop systems to be seamlessly integrated with the network.

AS-i can be used as a stand alone network or can be connected to a higher level system, such as DeviceNet or PROFIBUS-DP, through a gateway.  The gateway acts as a slave to the higher system and a master to the AS-i system.

Typical System Configuration

TURCK AS-interface System Description

Basic Parts List

A typical AS-i system consists of the following parts:

A = Master
B = AS-i Power Supply
C = AS-i Cable
D = AS-i I/O Modules (or Slaves)

AS-i stations require a network master (also called a scanner) to interface the stations to the host controller.  In some cases the scanner and controller are packaged as a single unit; in other cases the scanner acts as a gateway to a higher level network or to a PLC. TURCK AS-i stations are designed to be fully compatible with AS-i equipment from other manufacturers.

Communication Signal and Power

AS-i communication uses a Manchester encoded data signal, which results in a very noise immune system, even on the specified untwisted and unshielded cable.  The communication media is a simple two-wire cable.  Both power (30 VDC) for the stations and data are carried on the same wires.  This means the DC supply must be "decoupled" from the network (the power supply cannot "see" the AC data component on the wire).  Special AS-i power supplies are available, which incorporate this feature in a single package.  Optionally, a separate AS-i decoupling component (power conditioner) can be purchased, allowing the user to use a standard 30 VDC supply.

In many cases the AS-i communication power supply is not sufficient to power higher current outputs on the network.  In these cases most manufacturers make AS-i slaves which draw I/O current from a separate auxiliary supply.  The station electronics are still powered from the AS-i network.

Communication Rate/Cycle Time

AS-i communicates at a fixed data rate of 167 kbps.  The system’s cycle time is very predictable because of the simple communication scheme and fixed data rate.  For example, a network with 31 slaves will have a cycle time of less than 5 ms. A network with 62 slaves (all A and B addresses used) will have a cycle time of less than 10 ms.  If analog slaves are being used, the cycle time will change to account for the fact that an analog word takes multiple network cycles to transmit.

Maximum Ratings

The AS-i system uses a freeform layout topology.  Up to 100 m of cable can be used on a segment before a repeater or tuner needs to be installed to allow the network to be extended beyond the 100 m limit.  No terminating resistors are required.

Diagnostics

AS-i has limited field diagnostic capability, due to the limited amount of data transferred in each message. With v2.1, a peripheral fault bit can be reported by an AS-i station to indicate a fault with a field device.  This allows the user to easily determine the location of a system fault down to the station level. AS-i v3.0 has even more diagnostic capabilities, allowing asynchronous "mailbox" messaging to receive more detailed error information. Bihl+Wiedemann AS-i masters (provided by TURCK) provide comprehensive information about the status of each station on the network by using register based tables to display each occupied network address.

Addressing

The original AS-i system allowed only 4 bits of data to be transferred in each message for a fast and efficient data transfer system.  Slaves could be addressed from one to 31, but with the growth of the network more than 31 stations were often required.  Beginning with AS-i v2.1 stations were available with “AB” addressing.  This scheme allows the station to be addressed from 1A to 31A or 1B to 31B, with 62 total slaves with four discrete inputs and three discrete outputs each.  The extended address range (and the limitation to three outputs) is achieved by using one output bit as an AB address. When both A and B addressed slaves are on the same network, they are scanned on alternating cycles (first all the A slaves
are scanned, then all the B slaves).  Both AB and single-address slaves can be on the same network.  In this case the single-address (non AB style) slaves are scanned every cycle.  It’s important to note that not all v2.1 slaves use this addressing scheme, although it is often referred to as v2.1 addressing.

Analog Data

Although the original AS-i version only allowed discrete data transfer, v2.1 and higher support seamless analog data transfer. This is accomplished by sending a portion of the analog data on each of several consecutive network cycles; for example, a 16-bit word of data requires seven network cycles.  Further, AS-i v3.0 allows analog data to be transferred in a single cycle by consuming more than one address for the analog slave.