The digital link between an axis control module (master) and intelligent servodrives (slaves) is defined by European standard EN 61491 relative to electrical equipment for industrial machines.
The use of a distributed architecture enables the input/output applications (position encoder, emergency stop etc.) to be connected directly to the servodrives, which reduces cabling costs.
The fiber optic link enables high speed exchanges (2 or 4 MHz) and ensures immunity against electromagnetic interference.
There are two types of data exchanged via the digital link:
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cyclic data exchanged from master to slaves (position command etc.) or from slaves to master (position measurement etc.). Cyclic data exchange between the master and each slave is limited to 8 read and 8 write objects per SERCOS® cycle.
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non-cyclic data: complex commands, parameter write or read etc.
For each cycle, these exchanges are made via 2 reserved bytes in read and 2 reserved bytes in write. Several cycles are therefore required to exchange an object (e.g. to complete a parameter read).
Identification of Exchanged Objects
All objects may be accessed using an identification number: IDN.
The standard can identify 31 768 objects and specify about 300 (e.g. IDN 40 = speed value).
All objects consist of the following fields: Name (maximum 64 characters), attribute, unit, maximum value, minimum value, value.
The bus operating modes follow 5 phases as follows:
At start up
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Phase
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Operating mode
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Phase 0
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Bus ring test. The servodrives are in repeater mode.
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Phase 1
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Determination of slaves on the bus.
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Phase 2
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Servodrive system configuration.
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Phase 3
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Cyclic exchange programming.
Servodrive parametering.
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In normal operation
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Phase
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Operating mode
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Phase 4
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Active cyclic exchanges.
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Because each servodrive acts as a repeater on the bus, the system switches to phase 0 in the event of a power supply cut, a communication error, a fault on one of the servodrives or the disconnection of the power supply to the bus.
NOTE: Certain parameters (IDN) may only be accessed in phase 3 (see standard EN 61491). The following functions GetActualPhase, GetCommandedPhase and SetCommandedPhase indicate the active phase and set it in phase 3.
