Hi,

Have you placed __declspec before MBAP_Header too?
Yes:
Qt Code: 
  1. //#pragma pack(1)
  2. __declspec(align(1)) struct MB_Request
  3. {
  4.    uchar function_code;
  5.    quint16 start_adr;
  6.    quint16 quantity_regs;
  7. };
  8.  
  9. //#pragma pack(1)
  10. __declspec(align(1)) struct MBAP_Header
  11. {
  12.    quint16 transaction_id;
  13.    quint16 protocol_id;
  14.    quint16 len;
  15.    uchar unit_id;
  16. };
  17.  
  18. //#pragma pack(1)
  19. __declspec(align(1)) struct MB_Message
  20. {
  21.    MBAP_Header mb_hdr;
  22.    MB_Request mb_req;
  23. };
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But
Qt Code: 
  1. int iA1 = __alignof(MBAP_Header);
  2. int iA2 = __alignof(MB_Request);
  3. int iA3 = __alignof(MB_Message);
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Returns always "2" for iA1, iA2 and iA3.

Don't worry about it, using "#pragma pack(1)" is going well.

Thanks for your time and your knowdelege.