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MODBUS - MODBUS solution Print
Modbus is probably the most spread serial protocol in industry

Modbus solutions from ME Advise

MCS is the exclusive reseller of MODBUS slave solutions from ME Advise.

  

Modbus generals

Modbus is probably the most spread serial protocol in industry. Most devices, being able to communicate serial, talk Modbus.

The communication is master / slave driven. Modbus is capable to run full-duplex RS232 lines and half-duplex RS485 solutions. Also newer implementation variants run via Ethernet & TCP/IP.

One of the best sources for more detailed informationís is http://www.Modbus.org/

  

A typical Modbus telegram shown in hex characters is:

Request from master   02 03 00 08 00 01 05 FB

with the answer from slave 02 03 02 02 01 3C E4

Itís to slave 02 request 03, to send back from his address 0008, one word (00 01), with checksum 05 FB, and from slave 02 answer 03, with 02 bytes, the values 02 01, and checksum 3C E4. 

If you want to give your application an easy connectivity to most of the worlds devices, so you should choose Modbus at your first thought.

The application is purely BASCOM Basic. Itís a software implementation for Atmel AVR controllers. The smallest, fully tested version is for AVR 2313 with RS485, running the board you see below.

All here presented modules are the slave implementation, but you can easily change to your specific master implementation.

Modbus slave for AVR 2312

  

  

All written in pure BASCOM Basic.

Well / fully documented code.

' This is a empty shell to make the download possible.
' You must not compile this under no circumstances.
' It is only used for as loading shel and to discribe you how to use it.

 See Test Procedure Bellow
!!!!!!!!


' -----[ Program Description ]--------------------------------------------------
'
' This program implements the ModBus RTU protocol in an universal I/O device
' It uses a special designed HW or STK200 development kit for testing purpouse.
' It's a polled slave with 16 in and 16 outputs.
'
' -----[ Disclaimer ]-----------------------------------------------------------
'
' This example is offered on an "AS IS" basis, no warranty expressed or implied.
' The programers disclaim liability of any damages associated with the use of
' the hardware or software described herein.
' You use it on your own risk. We are not able to provide any free support.
'
' Copyright (c) 2003 Mike Eitel all rights reserved
'
' -----[ Revision History ]-----------------------------------------------------
'
' 030410 - Version AVRMODDIO16 Ver 1.50 slave mclib 2313 ERAM Mike Eitel

' *****[ conditional compiler flags ]*******************************************

Const Stk200 = 1 ' When 1 then STK200 else DTR103

' *****[ conditional TEST flag ]************************************************

$regfile = "2313def.dat"

' Define serial communication parameters
$baud = 9600 ' ModBus speed
$crystal = 4000000 ' Frequency of unit
' -----[ Aliases ]--------------------------------------------------------------


' -----[ Constants ]------------------------------------------------------------

' Telegramms solved
' This device is capable of max 4 bytes data transfer mode 03 & 16 !!!
'
' Request Read Holding Register(03) SS 03 SH SL PH PL CL CH
' Answer Read 16 Holding Register(03) SS 03 BC 1H 1L CL CH
' Answer Read 32 Holding Register(03)  SS 03 BC 1H 1L 2H 2L CL CH
'         
' DB( 1 2 3 4 5 6 7 8 9 )
'
' Request Preset Single Register(06) SS 06 RH RL 1H 1L CL CH
' Answer Preset Single Register(06) SS 06 RH RL 1H 1L CL CH
'        
' DB( 1 2 3 4 5 6 7 8 )
'
' Request Preset Multiple Register(16) SS 10 SH SL PH PL BC 1H 1L 2H 2L CL CH
' Answer Preset Multiple Register(16) SS 10 SH SL PH PL CL CH
'             
' DB( 1 2 3 4 5 6 7 8 9 10 11 12 13 )
'
' ' SS = Slave Address
' ' 03 = Command 03
' ' SH = Starting Address High
' ' SL= Starting Address Low
' ' PH = Nr. of Points High
' ' PL = Points Nr.(16 bits) Low
' ' BC = Byte Count
' ' 1H = Data 1 High
' ' 1L = Data 1 Low
' ' 2H = Data 2 High
' ' 2L = Data 2 Low
' ' CL = CRC Low
' ' CH = CRC High

' -----[ Initialization ]---------------------------------------------
'
' Define port parameters
' Port B Is Used as output to LED of STK200

' Port D is used for communication and for latch controll
' Ddrd.0 ' RXD
' Ddrd.1 ' TXD

' Ddrd.2 , 3, 4, 5 are usable switches of the STK200

' Ddrd.6 ' Enable for RS422 send pin
' Ddrd.7 ' NC for 2313


 Datas
(adressparamter) = Myaddress ' Set new Slave address
 Datas
(parameters) = Myparameter ' Set new Slave parameters

#if Stk200 ' ***** TEST MODE ***
 Datas
(adressparamter) = 2  ' Set test address
 Datas
(parameters) = 4 ' Set test parameters
#endif ' ***** TEST MODE ***


'------[ Program ]----------------------------------------------------
 
' Routine for Inputs
 Datas
(1) = ' Read allowed swiches

 
' Routine for counting input changes
 Datas
(7) = Datas(1) And Datas(parameters) ' Check only

 
' Routine for Outputs
 
' If datas(5) is requesting high input pin, force output high. When input
 
' reaches high level forcing is surpressed.
 
' Generally output is OR overwriten high by normal output setting datas(3)
 
Portb = Not [(datas(5) And Not Datas(1) ) Or Datas(3) ]

' And how to you make a test?
' 1. Place 2313 into STK200 and switch on
' 2. First you use the load to AVR without compiling
' 3. Choose your modbus master source and send to this slave with address 02

' In this description I talk from bytes, cause depending from used application
' U will use registers starting with register 0001. Sometimes the start with
' 10000 or higher. Please find that yourself, by monitoring the telegram.

' Inputs
' Test input switches by reading from bytes 00 00. Pressing the middle 4 switches
' will show a changed telegram status.

' The so called parameter is preset to monitor the activities of switch PD2.
' Each movement on that switch will increase value of bytes 00 06

' When you read from byte 00 0E you will see an incremented word counter.


' Outputs
' Test normal output functionality by sending to address 00 02. You can control
' result by LED status.

' Test fast output position control reaction by using bytes 00 04. As soon as
' you set a bit you will see LED going on, ( I propose PB4 ) and if you press
' according input ( PD4 ) the LED will switch off.

' Changing parameters
' If you write to byte 00 09 you can change slave address. And if you write to
' byte 00 08 you can change the surviewed switches. These changements are per-
' manetely stored in EERAM but are overwriten when device is powered up.

' This sample will stop working after ca. 3 minutes. You have to do reset AVR !!

' Have fun M. Eitel




  • Baudrate 9600 8,N,1. Change baudrate in code prepared
  • RS485 control included, RS232 also working.
  • Telegrams solved: 3, 6 16 = 16 bit word and 32 bit long register access possible.
  • Slave address remote programmable, stored in EERAM.
  • Always accepted default slave address: FE to set address and parameters.
  • Fully independent interrupt driven, send and receive communication.
  • Receive ring buffer.
  • Independent command detection buffer.
  • Independent send buffer.
  • 16 Byte Data Array from starting from R0001 implemented.
  • Time based I/O multiplexing included for 16 in and 16 outputs.
 

Programmable signal counter for first 8 input pins.

 

  

Running layout as base of your development available.

  

  

ME Advise

Me, Mike Eitel, Iím in industrial business since 1981 and have done a lot of jobs. When I came in contact with Atmelís AVR, I landed soon at BASCOM. And Iím using that compiler since August 1999 and [ as Iím too old to be a genius ;-)  ] Iím used to write a lot of comments.

  

Beside of the AVR, Iím a specialist for industrial solutions, especially when they need a control system with high level HMI, or most commonly called SCADA system. Made some nice implementations. For some years I sold a SW called Wizcon, distributed by Axeda www.axeda.com. Still using Wizcon, nothing was more logical, than making a connection between my AVR projects and that PC based visualization. For reasons I explained before, I choose Modbus as best protocol, realize and test my applications that way. I meantime I have a AVR family solution with 8515 with LCD and 8535 with PID regulators. The bigger processors run additionally multitasking. Works fine!

  

Support email: michael.eitel@schweiz.ch


Content

  

Modbus generals. 1  

Modbus slave for AVR 2312. 1  

ME Advise. 3  

Content  4  

Module / label explanation:  5  

Conditional compiler flags. 5  

Define serial communication parameters. 5  

Telegramms solved. 5  

Other parameters. 6  

Pseudo constants programmed via telegram.. 6  

Important Variables. 6  

Define port parameters. 6  

Setup interrupts. 7  

Transfer from ERAM slave address and parameters. 7  

_com_z:  7  

_com_1:  7  

_com_2:  8  

_com_4:  8  

Subroutine for sending values. 9  

_send:  9  

_again:  9  

Interrupt routine for preparing serial port  9  

Interrupt routine for reading serial input  10  

Interrupt routine for sendig serial output  10  

Interrupt routine for multiplexing In / Outputs. 10  


Module / label explanation:

Conditional compiler flags

$regfile = "2313def.dat"

This is compiled for AVR 2313

  

$lib "Modbus.lbx"

The checksum is calculated via marks routine

  

Const Test = 1  ' When 1 then Test modus

Allows working without existing I/O chips

  

Const Stk200 = 1 ' When 1 then STK200 else DTR103

This allows working with STK200 and RS232 without having the board.

Define serial communication parameters

$baud = 9600 ' Modbus speed

#if Stk200

 $crystal = 4000000

 Ubrr = 25 ' For 9600 baud

#else

 $crystal = 8000000

 Ubrr = 51 ' For 9600 baud

#endif

Telegramms solved

This device is capable of max 4 bytes data transfer in mode 03 or mode 16. Otherwise the receive and send buffers are to small

  

Two sorts of communication are solved:

  

Reading data in 16 bit and 32bit request via the telegram 03 and

Sending 16 bit data via telegram 06 and 32bit via telegram 16.

  

  

Request Read Holding Register(03)  SS 03 SH SL PH PL CL CH

Answer Read 16  Holding Register(03)  SS 03 BC 1H 1L CL CH

Answer Read 32  Holding Register(03)  SS 03 BC 1H 1L 2H 2L CL CH

          

  DB( 1  2   3  4  5  6  7  8  9 )

  

Request  Preset Single Register(06) SS 06 RH RL 1H 1L CL CH

Answer Preset Single Register(06) SS 06 RH RL 1H 1L CL CH

          

  DB( 1  2   3  4  5  6  7  8  )

  

Request  Preset Multiple Register(16) SS 10 SH SL PH PL BC 1H 1L 2H 2L CL CH

Answer Preset Multiple Register(16)  SS 10 SH SL PH PL CL CH

              

 DB( 1  2  3  4  5  6  7  8  9  10 11 12 13

  

SS = Slave Address

03 = Command 03 as example for a telegram type 03

BC = Byte Count. In some telegrams existing byte counter

SH = Starting Address High of the AVR data area

SL= Starting Address Low

PH = Number of Points High or general spoken number of requested word dataís

PL = Points Nr.(16 bits) Low

CL = CRC Low. Checksum according Modbus low nibble

CH = CRC High

1H = Data 1 High

1L = Data 1 Low

2H = Data 2 High

2L = Data 2 Low

Other parameters

Const Initializing = &HFE

The device will always react on this address. It is foreseen to make initial programming of device parameters and address: Myaddress.

  

Const Adressparamter = 10   ' Slaveparameters position

Const Parameters = 9   ' Slaveparameters position

These pointers define the position in the dataís volatile RAM array

  

Const Dataarea = 16  ' Databuffer + 2 Slaveparameters

This is the size of the data area.

  

Const Savetoeram = 10    ' Address of Myaddress

This pointers are used to define the position in EERAM.

  

Const Maxchar = 19  ' Nr. of characters for serial buffer

Size of the receive ring buffer

  

Const Telegrambytes = 14   ' Nr. of transmitted Bytes

Max length of the used telegrams. More important in the bigger devices like 8515.

  

Const Pause = 2  ' Wait time before answer

A small device like this AVR reacts to fast for a SCADA system. With this parameter you adjust a small wait time for the answer telegram.

Pseudo constants programmed via telegram

Dim Myaddress As Eram Byte At &H02   ' Address for this node (2-253)

Dim Myparameter As Eram Byte At &H03 ' Address for this node (2-253)

Important Variables

Dim Bytebuffer(maxchar) As Byte   ' Receive serial-buffer

Dim Db(telegrambytes) As Byte   ' Dataís in telegram

Dim Sdb(telegrambytes) As Byte   ' Dataís in send telegram

Dim Datas(dataarea) As Byte At &H60   ' Place to keep the dynamic dataís

Define port parameters

Port B Is Used For Data Exchange With The Latches

'Ddrb = &HFF ' WRITE to multiple I/O Data port

'Ddrb = &H00 ' READ from multiple I/O Data port

  

Port D is used for communication and for latch controll

Ddrd = &B01111100 ' Set Portd Pin 2..6 as Outputs

Ď Ddrd.0 ' RXD

' Ddrd.1 ' TXD

' Ddrd.2 ' Strobe Enable Read chip 1

' Ddrd.3 ' Strobe Enable Read chip 2

' Ddrd.4 ' Strobe output latches Allegro UCN 5801A 1

' Ddrd.5  ' Strobe output latches Allegro UCN 5801A 1

' Ddrd.6 ' Enable for RS422 send pin

' Ddrd.7 ' NC for 2313

  

Setup interrupts

Allow receive & transmit ISR

Config Timer0

Transfer from ERAM slave address and parameters

 Datas(adressparamter) = Myaddress ' Set new Slave address

 Datas(parameters) = Myparameter ' Set new Slave parameters

_com_z:

Startingpoint for telegrams

If received address is this node or the general initialization address, read next Byte from master, if not return to start

_com_1:

If next character is one of the three solved commands wait for next chars or restart

 Select Case Db(2)

 Case 3 : ' A read command

 Case 6 : ' A write command

 Case 16 : ' A write command

 Else com_z  '  Wrong command = restart

 Else ' Wait for next incoming data

_com_2:

Get telegram into binary mode

Clear the receive buffer to make a calculated response possible

Check the CRC of the message for errors

_com_4:

No CRC errors in packet so check what to do and start reading / writing

Select Case of telegram

Case 03 : ' READING datas 1..n

 ' Request Read Holding Register(03) SS 03 SH SL PH PL CL CH

 ' Answer Read 16 Holding Register(03) SS 03 BC 1H 1L CL CH

 ' Answer Read 32 Holding Register(03) SS 03 BC 1H 1L 2H 2L x x CL CH

 Gosub _send ' Give answer

 Goto _com_z ' All done, go back to Start

  

Case 06 : ' WRITING dataís 16 bit

 ' Request Preset Single Register(06) SS 06 RH RL 1H 1L CL CH

 ' Answer Preset Single Register(06) SS 06 RH RL 1H 1L CL CH

 Gosub _send ' Give answer

 Goto _com_z ' All done, go back to Start

  

Case 16 : ' WRITING dataís 1..n

 ' Request Preset Multiple Register(16) SS 10 SH SL PH PL BC 1H 1L 2H 2L CL CH

 ' Answer Preset Multiple Register(16) SS 10 SH SL PH PL CL CH

 Gosub _send ' Give answer

 Goto _com_z ' All done, go back to Start

 End Select

Goto _com_z ' Not identified command

Subroutine for sending values

Start CRC generate routine and calculate CRC for all sending bytes

Send Packet To Master , Including The Sync Byte

Interrupt routine for preparing serial port

On powerup it's treated once !

Interrupt routine for reading serial input

Read into serial ring buffer

Interrupt routine for sendig serial output

Send contents from buffer

Interrupt routine for multiplexing In / Outputs

Timer0_isr: ' Controlls the multiplexing of data versus I/O's

' Port D is used for communication and for latch controll

' Ddrd.2 ' Strobe Enable Read chip 1

' Ddrd.3 ' Strobe Enable Read chip 2

' Ddrd.4 ' Strobe output latches Allegro UCN 5801A 1

' Ddrd.5 ' Strobe output latches Allegro UCN 5801A 2

' Ddrd.6 ' Enable for RS422 send pin

  

Select Case

Case 0: ' READ 1 --- High Byte ---

Case 1: ' READ 2 --- Low Byte ---

Case 2: ' WRITE 3 --- High Byte ---

Case 3: ' WRITE 4 --- Low Byte ---

Additional features:

1. When slave address is FF means not yet programmed, a HW test is possible, by cause signal of input 1..16 is copied to output 1..16.

2. The one byte parameter defines by AND function with input 1..8 counts up a counter (bytes 07 and 08 ) at each signal change.

 

Order code : MODBUS-2313