AS-i is a system in which exchange management is ensured by a single master who calls in succession, by scanning the bus, each detected slave and awaits a response.
The serial communication frame transmits:
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4 data bits (D0 to D3), which are the image of inputs or outputs according to the nature of the interface.
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4 parametering bits (P0 to P3), which are used to set the operating modes of the interface.
P0 to P3 bits are used for "intelligent" devices, including AS-i ASIC (specific integrated circuit). Operation can be modified while it is running.
The address of the slave is coded on 5 bits.
At the request of the AS-i master, outputs are set and the inputs for AS-i devices are sent in the slave’s response.
Each slave connected to the AS-i bus must have an address between 1 and 31 (coding on 5 bits).
The slaves delivered from the factory have the address 0 (the address of the slave is memorized in a non-volatile format).
Addresses are programmed using a specialized addressing terminal.
NOTE: When replacing a faulty slave whose address has been set, the address of the slave to be replaced can be updated automatically.
All slave devices connected to the AS-i bus are identified by:
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An I/O Code (input/output distribution code).
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An identification code, which completes the functional identification of the slave.
These identifications allow the AS-i master to recognize the configuration, which is present on the bus.
These different profiles have been developed by the AS-i Association. They are used to distinguish between input, output and mixed modules, "intelligent" device families, etc.
Maximum Number of Inputs/Outputs
An AS-i bus can support a maximum of 31 slaves.
Each slave can have a maximum of 4 inputs and/or 4 outputs.
This makes it possible to manage a maximum of 124 inputs + 124 outputs, or 240 discrete inputs/outputs when all active devices have 4 inputs and outputs.
The AS-i cable is a twin-wire link on which communications and power supply for the connected devices are transmitted.
The link does not need to be twisted.
The cross-section of wires can be from 2 x 0.75 mm2, 2 x 1.5 mm2 or 2 x 2.5 mm2, according to the current consumed by the devices.
Topology and Maximum Length of AS-i Bus
The topology of the AS-i bus is with flexible and adapted to meet the users needs (point to point, on line, tree structure etc.).
In every case, the total length of all the branches of the bus must not exceed 100 meters without a repeater.
This is the cycle time between slave(s) and the TSX SAY 100 module.
The AS-i system always transmits information, which is the same length to each slave on the bus. The AS-i cycle time depends on the number of slaves connected to the bus.
In the presence of 31 functioning slaves, this time period will be a maximum of 5 ms.
Reliable operation is ensured by the transmission process used (Manchester current and coding modulation). The master monitors the line supply voltage and the data sent. It detects transmission errors as well as slave failures, and sends the information to the PLC.
Exchanging or connecting a new slave during operation does not disturb communications between the master and the other slaves.
