Modbus testing

[RGV250]

Hi,
I have a Modbus device I am considering using, I recall several people have communicated over \ modbus so I was wondering what software/hardware they used to test things out first.
I have USB / RS485 converter and using the serial communicator program but do not seem to be able to connect to the device.

Regards,
Bob

[ricardourio]

Hello Bob,

   My main projects uses RS485 with MODBUS protocol. How can I help you ?

Regards

Ricardo Urio

[RGV250]

Hi Ricardo,
At the moment I am just trying to use a terminal program and USB / RS485 converter to see if the device works and to set it up.
I have tried Modbus Poll, Realterm and the serial commumicator but no response. I have seen something being returned in Modbus Poll but not sure what it is as I cannot find what it is sending.

Bob 

[Pepe]

Sample

[RGV250]

Hi Pepe,
Thanks for the code but the issue I have at the moment is I do not have any circuit, just the device and a USB converter. I had hoped to make sure the device works first before looking at spending time building a board if the device does not work.
I have 2 devices and neither give the response I am expecting according to the (not very helpful) information I found. They do seem to connect but that is about it.

Regards,
Bob
 

[Giuseppe]

Some time ago I tried to control a commercial card via mod-bus protocol.
Below is a piece of code to take as a starting point
There was a sequence of bytes to send and receive between the master and the slave

Code Select Expand

'At 9600 baud it takes 1ms to transmit 1 byte (104uS per bit) approx
'We need to calculate the transmission time to evaluate the delay to be entered just below
'transmission.This is because if we return en_485 = 0 without having finished the TX it goes to RX and cuts the bytes to be transmitted

tx:

Select codes    'According to the function code I go to set the 5 and 6 Bytes to be transmitted

 Case = 01
 buf[4] = n_outputs.HighByte
 buf[5] = n_outputs.LowByte
 
 Case = 02
 buf[4] = n_inputs.HighByte
 buf[5] = n_inputs.LowByte
 
 Case = 05
 buf[4] = value_output.HighByte
 buf[5] = value_output.LowByte
 
End Select
 
buf[0] = add_slave
buf[1] = codes
buf[2] = indirizzo.HighByte
buf[3] = indirizzo.LowByte


crc_fine = 5
GoSub  Calcul_CRC      'Routine  crc ok

buf[6] = crc.LowByte
buf[7] = crc.HighByte

en_485 = 1
 HSerOut [buf[0],buf[1],buf[2],buf[3],buf[4],buf[5],buf[6],buf[7]] 
 
stai_qui:
If TXSTA1.1 = 0 Then stai_qui     'it stops until it has transmitted all the bytes

en_485 = 0                        'Re-enabled the Master serial reception (Pin RE e DE linked together Max485)
 
Clear buf                         ' i clean the buf i use for both Tx and Rx
buft = 0                          ' I clean the index of the buf
INTCON.7 = 1                      ' I enable the interrupt to receive response from the slave
 
DelayMS 10                        'To give the interrupt time to receive the 8 buf

'---------------------------------------------------------------------
rx:

If flag.1 = 1 Then   'I put flag.1 = 1 in the interrupt when I am sure I have received all 8 bytes             
                     ''or from buf [0] to buf [7]. I always use the array variable buf si for Tx and Rx             
     
HSerOut2 [buf[0],buf[1],buf[2],buf[3],buf[4],buf[5],buf[6],buf[7]]
DelayMS 3
flag.1 = 0   


    If buf[0] = add_slave And buf[1] = codes Then  'in the various response function codes I always have add_slave and codes

        Select codes

        Case = 01
        'n_byte_stato_output = buf[2]   ' to be developed
                                           
        Case = 05
       
        crc_fine = 5       'If the codes is 05 in the Calcul_CRC routine it must process the first 6 Bytes received in the interrupt
        GoSub Calcul_CRC
 
        If crc.LowByte = buf[6] And crc.HighByte = buf[7] Then   'compare the crc received from the slave with the calculated crc
            If buf[4] = $FF Then  testo="On"                     'with the first 6 Bytes always received by the slave
            If buf[4] = $00 Then  testo="Off"
            Cls
            Print At 1,1,"Rele-",Dec3 indirizzo,"-",testo
            DelayMS 3000
        End If

        Case = 15    ' to be developed

        Case = 02
        max = 3
        pirella = 0
        teresa = 0
       
        For gg = 0 To (buf[2]-1)           'In buf [2] I have the number of Bytes to receive which represent the status of the slave Inputs
        byte_input[pirella] = buf[max]     
        max = max + 1
        pirella = pirella + 1
       
        If pirella = 2  Then               'I put pirella in teresa when pirella is > 2
        teresa = pirella + teresa          '
        pirella = 0
        End If
       
        Next gg   
       
        teresa = pirella + teresa 
        crc_fine = teresa + 2    '2 are the Bytes that I always receive from 0 to 2: Add slave + codes + Byte indicating n_byte Input
        GoSub Calcul_CRC
       
        max_1 = 3 + teresa       'I calculate the place of the Low Byte of crc on the basis of the Bytes received in the buf
        max_2 = 3 + teresa + 1   'I calculate the place of the High Byte of crc on the basis of the Bytes received in the buf
       
        HSerOut2 [crc.LowByte,buf[max_1],crc.HighByte,buf[max_2],teresa,buf[2]]
        DelayMS 3
       
       
        If crc.LowByte = buf[max_1]  And crc.HighByte = buf[max_2] Then
       
           Select byte_input[0]                'I print on the display which inputs are active
           Case = 01
            Print At 1,1,"Input 1 active"
            DelayMS 3000
           Case = 02   
            Print At 1,1,"Input 2 active"
            DelayMS 3000
           Case = 04
            Print At 1,1,"Input 3 active"
            DelayMS 3000
           Case = 08
            Print At 1,1,"Input 4 active"
            DelayMS 3000
           
           Case = 05
            Print At 1,1,"Input 1-3 active"
            DelayMS 3000
           Case = 06
            Print At 1,1,"Input 2-3 active"
            DelayMS 3000
           Case = 07
            Print At 1,1,"Inp 1-2-3 active"
            DelayMS 3000
           Case = 09
            Print At 1,1,"Input 1-4 active"
            DelayMS 3000
           Case = 10
            Print At 1,1,"Input 2-4 active"
            DelayMS 3000
           Case = 11
            Print At 1,1,"Inp 1-2-4 active"
            DelayMS 3000
           Case = 12
            Print At 1,1,"Input 3-4 active"
            DelayMS 3000
           Case = 13
            Print At 1,1,"Inp 1-3-4 active"
            DelayMS 3000
           Case = 14
            Print At 1,1,"Inp 2-3-4 active"
            DelayMS 3000
           Case = 15
            Print At 1,1,"Inp all active"
            DelayMS 3000
           
           
           End Select
        End If   
    End Select
    End If
EndIf


GoTo main11

'------------------------------------------------------------------------------------------------------------------------------
'The crc represents a word variable that is used in the Modbus protocol to check if a correct one has occurred
'transmission or reception between a master and a slave. Usually 8-byte strings are sent between master and slave.
'The master based on the first 6 bytes of the string must calculate the crc and finally add the result in the 7 and 8 bytes to send
'everything to the slave Once the slave has received the 8 bytes, it will have to recalculate the crc using the first 6 bytes and compare the value
'with the crc calculated and sent by the master. If the 2 crc are the same, the command has been received correctly.
'Then subsequently the slave will have to execute the command and formulate the answer always with a crc calculated thanks to the first 6 bytes
'The master received the response will have to recalculate the crc of the response sent by the slave and compare the 2 crc if
'are equal the reception of the answer is correct.

Calcul_CRC:

crc=$FFFF                 
For a = 0 To crc_fine     
                         
  crc = crc^buf[a]       
  For j = 1 To 8         
   If crc.Bit0 = 1 Then
   crc =$A001^(crc>>1)   
   Else
   crc = crc >>1           
   EndIf
  Next j
Next a
Return

'-----------------------------------------Interrupt Routine---------------------------------------------------------------
On_Interrupt GoTo int_routine

int_routine:

Context Save

If PIR1.5 = 1 Then     'PIR1.5 is the Bit 5 of the PIR1 register is the flag of the Interrupt occurred for rx EUSART
                     
 Clear PIR1.5          'With Clear I bring the PIR1.5 to 0 so that other interrupts can occur

 buf[buft] = RCREG1     'RCREG1 register where the Eusart 1 reception data arrive                                       
 Inc buft
 
 If buf[1] = 02 And buf[2]<> 0 And flag.2 = 0 Then   '
   flag.2 = 1
   buft_end = buf[2] + 2 + buft - 1   ' the number 2 represents the Byte crc. Buf [2] indicates the Byte Input slave number
 End If
 
  If buft > buft_end Then    'buft_end = 7 then If there is buf [1] = 02 it can vary according to the Bytes it must receive
   flag.1 = 1
   flag.2 = 0
   buft = 0
   buft_end = 7
   INTCON.7 = 0
  End If
 'Clear PIR1.5
EndIf

Context Restore       
                   



[Yasin]

Have you tried the computer program "Modbus Poll"?

[RGV250]

Hi Yasin,
I am trying Modbus Poll, the problem I have is there seems to be a response but I have no idea what is being sent as it does not tie up with the data?? sheet.
If I select Display / Communication there is supposed data coming back but I cannot find out where in the program the data being sent is so I cannot figure our what to try to send. I am also getting 03 Illegal value but the program seems pretty poor to me as there is no indication which value it does not like.

Regards,
Bob

[Yasin]

Hi @RGV250 . Some modbus devices do not recognize the "H03" function code. Can you try using "H04" as function code.

[RGV250]

Hi,
Thanks for all the replies, I have just found Simply Modbus https://www.simplymodbus.ca/RTUmaster.htm which is a lot clearer to use and I can now see what it is sending and I am now getting the responses I was expecting.

Bob