Commissioning Promag 53 PROFIBUS DP/PA
124 Endress+Hauser
6.8 PROFIBUS DP/PA acyclic data transmission
Acyclic data transmission is used to transmit parameters during commissioning or
maintenance, or to display additional measured variables that are not included in cyclic data
traffic. Thus parameters for identification, control or adjustment in the various blocks
(physical block, transducer block, function block) can be changed while the device is in the
process of cyclic data transmission with a PLC.
The measuring device supports the two basic types of acyclic data transmission:
• MS2AC communication with 2 available SAP´s
• MS1AC communication
6.8.1 Master class 2 acyclic (MS2AC)
MS2AC is acyclic data transmission between a field device and a Class 2 master (z.B.
FieldCare, Siemens PDM etc. → 74). During this process, the master opens a
communication channel via an SAP (Service Access Point) to access the device.
All parameters to be exchanged with a device via PROFIBUS must be made known to a Class
2 master. This assignment to each individual parameter takes place either in a device
description (DD), a DTM (Device Type Manager), or inside a software component in the
master via slot and index addressing.
When using MS2AC communication, note the following:
• As described above, a Class 2 master accesses a device via special SAP´s.
Therefore, the number of Class 2 masters that can communicate with a device
simultaneously is limited to the number of SAPs available for this data transmission.
• The use of a Class 2 master increases the cycle time of the bus system. This must be taken
into account when programming the control system used.
6.8.2 Master class 1 acyclic (MS1AC)
In MS1AC, a cyclic master, that is already reading the cyclic data from the device or writing
to the device opens the communication channel via the SAP 0x33 (special Service Access
Point for MS1AC) and can then read or write a parameter acyclically like a Class 2 master via
the slot and the index (if supported).
When using MS1AC communication, note the following:
• Currently, few PROFIBUS masters on the market support this data transmission.
• MS1AC is not supported by all PROFIBUS devices.
• In the user program, note that constant writing of parameters (for example, at every cycle
of the program) can drastically reduce the service life of a device. Acyclically written
parameters are written to voltage-resistant memory modules (EEPROM, Flash etc.). These
memory modules are designed for a limited number of write operations. In normal
operation without MS1AC (during parameter configuration), the number of write
operations does not even come close to this number. If programming is incorrect, this
maximum number can be reached quickly, drastically reducing a device's service life.
!
Note!
The memory module of the measuring device is designed for one million write operations.
To Next Page
Loading...
