ModBus Master control script commands

ModBus tutorial

ModBus script commands are stored as table entries in NVEM page 0. In the following pages, each command consists of a 1-byte numeric command code followed by 0 to several parameters. 16 bit parameters have '16' as part of their name. Other parameters are 8-bit.

Command codes less than 128 map to the same-numbered ModBus function. Codes 128 and over provide internal functions.

Here are a list of equates in SPLat format:

;--MODBUS function codes
MBkFunc_ReadCoils                EQU   1              ;ReadCoils,    SlaveAddr,  #SrcAddr,   ItemCount,   #SPLatRAM16
MBkFunc_ReadDiscreteInputs       EQU   2              ;ReadInputs,   SlaveAddr,  #SrcAddr,   ItemCount,   #SPLatRAM16
MBkFunc_ReadHoldingRegisters     EQU   3              ;ReadHReg,     SlaveAddr,  #SrcAddr,   ItemCount,   #SPLatRAM16, NumFormat
MBkFunc_ReadInputRegisters       EQU   4              ;ReadInputs,   SlaveAddr,  #SrcAddr,   ItemCount,   #SPLatRAM16, NumFormat
MBkFunc_WriteSingleCoil          EQU   5              ;WriteCoil,    SlaveAddr,  #DstAddr,   #SPLatRAM16
MBkFunc_WriteSingleRegister      EQU   6              ;WriteHReg,    SlaveAddr,  #DstAddr,   #SPLatRAM16, NumFormat
MBkFunc_WriteMultipleCoils       EQU   15             ;WriteCoils,   SlaveAddr,  #DstAddr,   ItemCount,   #SPLatRAM16
MBkFunc_WriteMultipleRegisters   EQU   16             ;WriteMultReg, SlaveAddr,  #DstAddr,   ItemCount,   #SPLatRAM16, NumFormat
MBkFunc_Goto                     EQU   128            ;Goto          NV0Ptr Addr
MBkFunc_Stop                     EQU   129            ;Stop
MBkFunc_Delay                    EQU   130            ;Delay,        10msTicks (0 - 255)
;--MODBUS number format codes
;for float labels, this is what the letters mean A=31:24, B=23:16, C=15:8, D=7:0
MBkFormat_Byte                   EQU   0              ;only save LSB of big endian register (hence save the second byte) (standard)
MBkFormat_Word                   EQU   1              ;16 bit big endian
MBkFormat_ByteLE                 EQU   2              ;byte, little endian (non-standard)
MBkFormat_WordLE                 EQU   3              ;word, little endian (non-standard)
MBkFormat_FloatABCD              EQU   4              ;big endian
MBkFormat_FloatBADC              EQU   5              ;big endian, byte swapped
MBkFormat_FloatDCBA              EQU   6              ;little endian
MBkFormat_FloatCDAB              EQU   7              ;little endian byte swapped (Modicon and Wonderware standard)

And an example of usage

   ;#Open_Serial Port( COM0 ) MODBUSMaster( 38400, n, 0, 50 )
   COMRunScript   pMODBUSScript


   NVEM0                                              ;Start of non-volatile memory

pMODBUSScript
   NV0Byte  MBkFunc_ReadCoils, 0, #160, 8, #asCoils

   NV0Byte  MBkFunc_WriteSingleCoil, 0, #1008, #sSingleCoil

   NV0Byte  MBkFunc_ReadHoldingRegisters, 0, #100, 1, #wCount, MBkFormat_Word

   NV0Byte  MBkFunc_ReadHoldingRegisters, 0, #103, 6, #fCount, MBkFormat_Word

   NV0Byte  MBkFunc_WriteSingleRegister, 0, #115, #wCount, MBkFormat_Word

   NV0Byte  MBkFunc_WriteMultipleRegisters, 0, #117, 6, #fCountIEEE_BER, MBkFormat_FloatCDAB

   NV0Byte  MBkFunc_Delay,10

   NV0Byte  MBkFunc_Goto
   NV0Ptr   pMODBUSScript