Contents Up Previous Next

Special requirements with Profibus

The following must be observed with Profibus PDV0: The maximum length of a data packet transferred from the master to a module is 8 bytes. This is sufficient for the proper control of the module (maximum 7 bytes are required for a message from the master to the module).

Figure 1.3: Profibus data frame

The maximum length of the data packet sent from the module to the master (reply) is limited to 16 bytes (GSD file). To send / receive larger data packets, you might need to use fragmentation . With 16 bytes, the longest message from the module to the master occurring during normal operation (14 bytes) can be catered for. The remaining 2 bytes that are always found at the end of the Profibus message (bytes 14 and 15) indicate

  1. the current state of the module (byte 14) and
  2. command counter (MsgCount) (byte 15)
(In fragmented messages, these two bytes are used for data.)

Only the high 8 bits of the status word are written. The error code is omitted. For errors, Profibus offers extended diagnostics. The error code is included in the output data.


For messages sent by the master to the module, a response is sent and the MsgCount is incremented by 1. This ensures that each request is acknowledged, event if there are impulse messages.
Impulse messages do not increase the MsgCount!


If the position in which the module is currently found is to be achieved, the module replies with "command received" followed instantly with "position reached" in the next Profibus cycle. As the control system connected with the Profibus might not query data with each Profibus cycle, the acknowledge messages might be lost during the motion command. The MsgCount ensures that an acknowledgment of the request is received. The status byte (byte 14) contains up-to-date information regarding the status of the module.

The last bit of the MsgCount can be evaluated as a toggle bit (module to master messages.). For data transfer from the master to the module, the not yet used byte 8 can be used as the toggle byte, or bit 63 can be used as toggle bit.


Groups are fully supported by the SYNC, FREEZE mechanism implemented in Profibus.
Addresses can be changed at any time with the "Set Slave Address'' (SAP 55) service. "Real No Add Change'' is stored in the group byte gespeichert. A set "Real No Add Change'' (0xFF) can thus be deleted by reconfiguring the group byte.

If consistent data transfer is not possible, the module can be operated as follows:

  1. Use SYNC, UNSYNC mechanism.
  2. Set D-Len to "0". Fill up all data and set D-Len when all data is added.