Examples for CTT2 Write Request/Response and Read Request/Response services

Access to the drive parameters of the decentralized inverter via the CTT2 "Write Request/ Response" and "Read Request/Response" services is explained in more detail with a few examples.

The following figure shows the general process:

[1]

PLC

[2]

AS‑Interface master

[3]

B device

[4]

Write Request message – This is sent to the B device with the parameter access data.

[5]

Cyclic data exchange

[6]

Acyclic parameter communication from the AS-Interface master to the B device consisting of:

  • 3 byte CTT2 service
  • 12 byte SPA Request message
    • for read access with the SPA Read service
    • for write access with the SPA Write service

[7]

Write Response message – This is sent directly after the Write Request message has been transmitted to the AS‑Interface master.

[8]

Acyclic parameter communication from the B device to the AS-Interface master consisting of 1 byte CTT2 service

[9]

Write Response message – Contains the confirmation of the transmission, but not the result of the parameter access.

[10]

Read Request message – Contains the query for the result of the parameter access.

[11]

Acyclic parameter communication from the AS-Interface master to the B device consisting of 3 bytes CTT2 service

[12]

Read Response message – This is sent to the AS-Interface master after the SPA service has been processed.

[13]

Acyclic parameter communication from the B device to the AS-Interface master consisting of:

  • 1 byte CTT2 service
  • 12 byte SPA Response message

[14]

Read Response message – Contains the result of the parameter access in the SPA Response message.

In the event of a faulty execution of the CTT2 service, the corresponding Error Response message will be returned. For example:

  • If the Read Request message is sent before the Write Response message has been received, CTT2 error 5 (last request not confirmed) will be returned in the Error Response message.
  • If the Read Request message is sent before the SPA service in the inverter has been fully executed, CTT2 error 4 (request not fully completed) will be returned in the Error Response message.

For information regarding error causes and measures for fault elimination, refer to chapter CTT2 service errors in Error Response messages.

Additional information