Bihl+Wiedemann, considered the "AS-I masters”, is the leading supplier of AS-I master and gateway products. Their broad product range enables users to select from a wide variety of higher level fieldbuses or PC/PLC control solutions. Additionally, Bihl+Wiedemann provides a wide variety of analog AS-I slaves, PC-board level devices for OEMs and sophisticated AS-I accessory products. TURCK has partnered with Bihl+Wiedemann to distribute and support their products in North America.
TURCK offers a complete line of molded AS-I cordsets to facilitate network installation, resulting in a faster start-up and fewer wiring errors. The bus and drop cables are specially designed foil-shielded, high-flex cables with very low inductance and capacitance to minimize propagation delay time. AS-I cables consist of a single untwisted and unshielded wire pair that carries both 30 VDC power and the network data. AS-I was originally designed for use with flat cable using an insulation displacement connection technology, but the use of round cables with sealed connectors has become more common. TURCK provides both cable options.
AS-I has limited field diagnostic capability, due to the limited amount of data transferred in each message. Although 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 provide comprehensive information about the status of each station on the network by using register based tables to display each occupied network address.
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 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.
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.
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.
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.
AS-interface Masters and Gateways
AS-i networks can be controlled by stand-alone "masters" or by "gateways" to higher-level networks. The terms “master” and “gateway” as used here differ in the following way: A master is an AS-i controller that provides a direct link to the host (PLC, PC, DCS etc.); a gateway is an AS-i master, while also being a slave to a higher-level system (such as DeviceNet, PROFIBUS-DP or Ethernet). In the case of a gateway, the AS-i information is compiled by the AS-i master and communicated through the higher-level system as a standard slave data map. Gateways are often used to incorporate the flexibility of an AS-i system into an already planned or existing system using a different, higher-level system.