MODBUS Telegrams

The send and receive of telegrams shown in this section are not visible in the PLC. However, the complete telegrams can be made visible using a serial data analyzer connected to the connection line between server and client, if required.

The amount of user data depends on the capabilities of the server and the client.

For the following examples, it is assumed that one AC500 MODBUS module is used as client and another one is used as server. There may be different properties if modules of other manufacturers are used.

• FCT 1 or 2: Read n bits
  • Example
• FCT 3 or 4: Read n words
  • Example
• FCT 3 or 4: Read n double words
  • Example
• FCT \ 5: Write 1 bit
  • Example
• FCT \ 6: Write 1 word
  • Example
• FCT 7: Fast reading the status \ byte of the CPU
  • Example
• FCT 15: Write n bits
  • Example
• FCT \ 16: Write n words
  • Example
• FCT \ 16: Write n double words
  • Example
• FCT 22: Mask write register
  • Example
• FCT 23: Read/Write n words
  • Example
• Exception response by server
  • Example