Selection of the Called Slaves¶
Group assignment is part of the PROFIBUS DP slave configuration procedure and will be loaded to the PROFIBUS DP master as part of the slave parameter set. As default a slave is not connected to a specific group. This has to be done manually. The group assignment parameter is coded of type byte and provides a bit field. Each bit points to a certain group and several bits can be set in parallel. Thus a slave can be assigned to multiple groups at the same time which is limited to 8 groups in total.
| G8 | G7 | G6 | G5 | G4 | G3 | G2 | G1 |
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
For instance, if a slave is to be assigned to the groups 7 and 1, the master sends a byte with the decimal value 65 (= binary value 01000001) to this slave.
The Function Block inputs SLV and GROUP_SEL specify which slaves are to be called by a global control command during runtime.
SLV = 0..126 calls only the slave with a bus address = SLV, independent of the group assignment and of the value applied at input GROUP_SEL. All other slaves discard this telegram. Applying SLV = 127 and GROUP_SEL = 0 calls all slaves connected to the bus, independent of their group assignment. The group assignment defined during the configuration is only considered by the slaves if they receive a global control command with SLV = 127 and GROUP_SEL unequal to 0. In this case, each slave compares the GROUP_SEL value with the group assignment byte received during parameterization. The slave accepts the command if a bit-wise collation of these two values delivers a result unequal to 0 and discards the command if the collation delivers a value of 0. For instance, if a slave which is assigned to groups 1 and 7 (see above) receives a global control command with SLV = 127 and GROUP_SEL = 64 dec. (= 01000000 bin.), the command is also (among others) addressed to this slave.
| Group assignment | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 1 | |
| GROUP_SEL | AND | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 |
| Result of the comparison | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 |
The following table shows the possible combinations of the three parameters SLV, GROUP_SEL and group assignment and lists the slaves called with these combinations.
| SLV | GROUP_SEL | Group assignment | Called slaves |
| 0…126 | X | X | Only slave with bus address = SLV |
| 127 | 0 | X | All slaves |
| 127 | 1 - 255 | 1 - 255 | Slaves, for which a bit-wise collation of group assignment and GROUP_SEL delivers a value unequal to 0. |
Note
Group assignment does not have impact on cyclic data exchange.