Writing a parameter cyclically
For the cyclic transmission method, the handshake bit has to be changed to activate service processing (execution of WRITE service). When using acyclic PDU types, every inverter processes every request telegram and in this way always executes the parameter channel.
Parameters are set as follows:
- Enter the index of the parameter to be written in byte 2 (index high) and byte 3 (index low).
- Enter the data to be written in bytes 4 - 7.
- Enter the service identifier and the data length for the write service in the management byte (byte 0).
- In cyclical PDU types, the WRITE service is not transferred to the inverter until the handshake bit is changed. In acyclical PDU types, the parameter channel is always evaluated.
The inverter now processes the write service and sends the service confirmation back by changing the handshake bit.
Byte 0: Management | |||||||||||||||||
7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | ||||||||||
0 | 0/1 | 1 | 1 | 0 | 0 | 1 | 0 | ||||||||||
| Service identifier: 0010 = Write | ||||||||||||||||
| Data length: 11 = 4 bytes | ||||||||||||||||
| Handshake bit: Must be changed with every new job. | ||||||||||||||||
Status bit: 0 = No error while executing service 1 = Error while executing service | |||||||||||||||||
0/1 = Bit value is changed | |||||||||||||||||
The data length is 4 bytes for all parameters of SEW frequency inverters.
Using the WRITE service as an example, the following figure represents a process of setting parameters between the controller and inverter using a cyclic PDU type. To simplify the sequence, only the management byte of the parameter channel is shown here.
The frequency inverter only receives and returns the parameter channel while the controller is preparing the parameter channel for the write service. The service is not activated until the moment when the handshake bit is changed (in this example, when it changes from "0" to "1"). The frequency inverter now interprets the parameter channel and processes the write service; however, it continues to respond to all telegrams with handshake bit = 0. Confirmation that the service has been executed occurs when the handshake bit in the response telegram of the frequency inverter is set to the same value. The master now detects that the received handshake bit is once again the same as the one which was sent. It can now prepare another parameter setting procedure.
