54 Process Image WAGO-I/O-SYSTEM 750750-644 Bluetooth® RF-TransceiverManualVersion 2.0.0, valid from FW/HW Version 03/03Whether process data is overlaid by data from register or Mailbox data or processdata is made invalid is controlled by specific bits in the control byte (see section“Control/Status Byte (C/S Byte)”).6.3.1 MailboxThe 750-644 I/O module provides a protocol referred to as “Mailbox” below forparameterization and diagnostics. The protocol allows data exchange between the750-644 I/O module and a PLC application.The Mailbox of the 750-644 I/O module is not a permanent part of the processimage, but may be mapped as required. If the Mailbox is mapped, part of theprocess image is no longer available for exchanging process data with remotedevices.If bit 5 is set in the control byte in the output process image (PIO), it is viewed asa request to the 750-644 I/O module to map the Mailbox. The 750-644 I/Omodule responds to the request by setting bit 5 in the status byte. The Mailbox isthus considered mapped. If the Mailbox is mapped, it can be used to parameterizethe I/O module or to query status or diagnostic information.The Mailbox begins after the control/status byte and the internal byte with the 3rdbyte of the process image and, depending on the size set, covers 6, 12 or 18 bytesof the process image (see section “Process Image” > ... > “Process Data Overlaidby Mailbox”).The Mailbox size determines which mailbox commands can be executed!To configure with WAGO-I/O-CHECK or function blocks of the WAGO-I/O-PRO CAA, you can choose each available Mailbox size independent of fieldbuslimitations.• For successful configuration, a Mailbox size of at least 12 bytes isnecessary.• If all diagnostic commands are to be fully available, a Mailbox size of 18bytes is required.If you are using a fieldbus over which less than 20 bytes per data element can betransmitted (e.g. CANopen), you should reduce the Mailbox size again to anappropriate size after successfully completing the device configuration.In the area of the process image overlaid by the Mailbox, the data is interpreted asMailbox data, so commands (Opcodes) can be sent here. The structure of the datain the Mailbox is always identical:
