Endress+Hauser Gammapilot M FMG60, Руководство по эксплуатации

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Operation Gammapilot M FMG60 PROFIBUS PA
42 Hauser
5.6.4 Acyclic data exchange
Acyclic data exchange allows device parameters to be changed independently of the
communication between the device and a PLC.
Acyclic data exchange is used
• to transmit device parameters during commissioning and maintenance;
• to display measured values that are not acquired in cyclic traffic.
There are two types of acyclic data exchange:
Acyclic communication with a Class 2 master (MS2AC)
In the case of MS2AC, a Class 2 master opens a communication channel via a so-called
service access point (SAP) in order to access the device. Class 2 masters are for example:
• FieldCare
• PDM
Before data can be exchanged via PROFIBUS, however, the Class 2 master must be made
aware of the parameters contained within the field device. This can be done by:
• a device description (DD)
• a device type manager (DTM)
• a software component within the master, which accesses the parameters via slot and index
addresses.
NOTICE
Communication
‣
The DD or DTM is supplied by the device manufacturer.
‣
The number of Class 2 masters that can simultaneously access a device, is determined by
the number of SAPs that the device can provide. The number of SAPs varies from device
to device.
‣
The use of a Class 2 master increases the cycle time of the bus system. This must be taken
into consideration when the control system or PLC is programmed.
Acyclic communication with a Class 1 master (MS1AC)
In the case of MS1AC, a Class 1 master that is already communicating cyclically with a device
opens a communication channel via SAP 0x33, a special access point for MS1AC. As is the
case for a Class 2 master, the parameter is read or written via the slot and index.
NOTICE
MS1AC
‣
Up to now, only a few PROFIBUS masters on the market support MS1AC.
‣
Not all PROFIBUS field devices support MS1AC.
CAUTION
!
Permanent writing of parameters, e.g. with every cycle of the application program,
must be avoided, since this can drastically reduce the life of the device.
Acyclic write parameters are stored electrically in the RAM (EEPROM, Flash...). The
RAM modules are design for a limited number of write operations only. In standard
operation without MS1AC, i.e. during configuration of the device, the number of write
operations is negligible when compared to the limit. If the application program is badly
designed, however, this limit can be reached quickly, This would dramatically reduce
the lifetime of the device.