$lib "modbus.lbx"
$regfile = "m16def.dat"                                     ' specify the used micro
$crystal = 11059200                                         ' used crystal frequency
$baud = 9600                                                ' use baud rate
$hwstack = 32                                               ' default use 32 for the hardware stack
$swstack = 10                                               ' default use 10 for the SW stack
$framesize = 40                                             ' default use 40 for the frame space


'for writing a register Modbus_slave number 3 was used
'Const Modbus_slave = 3 ' for a write test
Const Modbus_slave = 2                                      ' for a read test

On Urxc Urxc_isr
Enable Urxc
Enable Interrupts
'dimension a buffer for the modbus data
Dim Mbuf(8) As Byte , Modc As Byte , Modt As Byte , Modw As Word
Dim Xx As Byte
Config Print0 = Portb.4 , Mode = Set
Config Pinb.4 = Output                                      'set the direction yourself
Config Portc = Output
Config Porta = Output




Do
  nop
  'add you main code here
Loop



'-------- interrupt handler for the URXC interrupt ----------------------------
Urxc_isr:
  sbis usr,7
  rjmp urxc_isr                                             ' wait until we can read the byte
  Modt = Udr                                                ' get the byte

  If Modc = 0 Then                                          'if we do not have any data yet

    If Modt = Modbus_slave Then                             'is this the slave adress?
        Modc = Modc + 1                                     'increase the buffer counter
        Mbuf(1) = Modt                                      'store address in location 1
    End If
  Elseif Modc = 1 Then                                      'we received the address already
    If Modt = 6 Or Modt = 3 Then                            'this demo works with function 6 and 3 only
       Mbuf(2) = Modt                                       'store the function
       Modc = Modc + 1                                      'increase buffer counter
    Elseif Modt = Modbus_slave Then                         'test if it is the slave number
       Mbuf(1) = Modt                                       'store it in that case
    Else
       Modc = 0                                             'reset it
    End If
  Else                                                      'we received the address and the function
    Modc = Modc + 1 : Mbuf(modc) = Modt                     ' store the data
    If Modc = 8 Then                                        'we need 8 bytes

       Modw = Makeint(mbuf(7) , Mbuf(8))                    ' create word of reeived crc
       If Modw = Crcmb(mbuf(1) , 6) Then                    ' does the crc match?

          If Mbuf(2) = 6 Then                               ' function 6 to write data
              Gosub 1
             'do something here like setting a port
             'portD = mbuf(6) ' lsb of received data
             Printbin Mbuf(1) ; 8                           'just reply the received buffer
          Elseif Mbuf(2) = 3 Then                           ' function 3 to read data
             Mbuf(3) = 2                                    ' byte count
             Mbuf(4) = 0                                    'MSB
             Mbuf(5) = Pinc                                 ' LSB return input of port D
             Modw = Crcmb(mbuf(1) , 5)                      ' create checksum
             Mbuf(6) = Low(modw)                            'add to buffer
             Mbuf(7) = High(modw)
             Printbin Mbuf(1) ; 7                           ' write response
          End If
       End If
       Modc = 0
    End If
  End If
Return

1:
  Portc = Mbuf(6)
 Porta.0 = Portc.7
 Porta.1 = Portc.6
 Porta.2 = Portc.5
 Porta.3 = Portc.4
 Porta.4 = Portc.3
 Porta.5 = Portc.2
 Porta.6 = Portc.1
 Porta.7 = Portc.0
Return