In an AS-Interface system, exchange management is provided by a single master which, by scanning the slaves on the bus, calls each slave in succession and awaits a response.
At the request of the AS-Interface master:
The communication series frame for slaves with standard addresses is:
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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 parametrizing bits (P0 to P3), which are used to set the operating modes of the interface
The communication series frame for slaves with extended addresses is:
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3 or 4 data bits, which are the image of inputs (4 bits, D0 to D3) or outputs (3 bits, D0 to D2) depending on the type of interface
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3 parameter bits (P0 to P2), which are used to set the operating modes of the interface
The Px bits are used for "intelligent" devices, including AS-Interface ASICs (Application-Specific Integrated Circuits). An operation can be modified while the slave is running.
Some slaves have an "ID1 code", which defines the internal functions of the slave.
NOTE: The base frame is the same for Analog slaves and Discrete slaves.
There is ascending compatibility between standard and extended addressing. This means that all slaves compliant with AS-Interface Standard Version 2.0 on the market are supported by BMX EIA 0100.
Each standard addressed slave connected to the AS-Interface bus must have an address between 1 and 31 on bank A.
Each extended addressed slave connected to the AS-Interface bus must have an address which lies between 1 and 31, either on bank A or on bank B.
The slaves are delivered from the factory with an 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 an inoperative slave whose address has been set, the address of the slave to be replaced can be updated
automatically.
All standard addressed slave devices connected to the AS-Interface bus are identified by:
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An IO code (input/output distribution code)
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An identification code (ID ), which completes the functional identification of the slave
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For Analog slaves ID1 code indicates the analog channel number for the slave
All extended addressed slaves connected to the AS-Interface bus are identified by:
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An IO code (input/output distribution code),
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An ID code that is always equal to “A”
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An ID1 code which defines the internal functions of the slave,
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An ID2, which completes the internal function identification of the slave.
These identifications allow the AS-Interface master to recognize the configuration that is present on the bus.
The different profiles have been developed by the AS-Interface Association. They are used to distinguish between input, output and mixed modules, "intelligent" device families, etc.
NOTE: A standard slave cannot be installed at an address in bank A if there is already a extended slave at the same address in bank B.
Maximum Number of Inputs/Outputs
For Discrete slaves on the same bus, an AS-Interface bus can support a maximum of either:
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31 standard addressed slaves, each with up to four-inputs and four-outputs, with addresses from 1 to 31.
Using standard addressed slaves, it is possible to manage a maximum of 124 inputs + 124 outputs, or 248 discrete inputs/outputs when all active devices have 4 inputs and 4 outputs.
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62 extended addressed slaves with 4I and/or 3O, using addresses from 1A / 1B to 31A / 31B.
Using extended addressed slaves, it is possible to manage a maximum of 496 inputs + 496 outputs (thus 992 inputs/outputs) when all active devices have 4 inputs and 3 outputs.
For Analog devices on the same bus, an AS-Interface bus can support a maximum of 124 Input and 124 Output channels.
The AS-Interface cable is a twin-wire link on which communications and power 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 AWG 18), 2 x 1.5 mm2 (2 x AWG 15) or 2 x 2.5 mm2 (2 x AWG 13), according to the current consumed by the devices.
Topology and Maximum Length of AS-Interface Bus
The topology of the AS-Interface bus is flexible. It can be adapted to meet the user network type (point to point, in line, tree structure etc.).
NOTE: The total length of all the branches of the bus must not exceed 100 meters unless a repeater is used. A maximum of 2 repeaters can be used for a cable maximum of 300 m.
Operation is provided by the transmission process used (Manchester current and coding modulation). The bus master monitors the line supply voltage and the data sent. It detects transmission and slave anomalies. It then sends this information to the PLC.
Exchanging or connecting a new slave during operation does not disturb communications between the bus master and the other slaves.
