Do
    Do 
        str_in = str_in & Link.ReadExisting
    Loop While ((str_in.Length < length_predicted) And ((DateAndTime.Now.Ticks - tmr_indicator) < lng_length))
    ....
    'Checking FCS code
    'Checking PLC Response code (No Command Error)
    ...    
    'Assume we're waiting for frame(s)
    'The matter that must be checked is the penultimate and the last
    'character received from PLC.
    if mid$(str_in,str_int.length - 1,1)<>"*" then 'Means some frame(s) are ready to be received
        'First Process Data 
        'Clear RS232 Input Buffer
        str_out = Link.readExisting
        'str_out = ""
        link.Write(vbCr)'Indicates Host Computer is ready to receive data
        ...
    end if
    ...
Loop While loop_condition