ModbusRTU

Latest Version: 1.0.2

This library creates and parses Modbus Protocol Data Units (PDU). It depends on Electric Imp’s CRC16 library to calculate the CRC-16 value of a string or blob.

Note You will not usually work with this library directly, but load it as a dependency for one of our other Modbus libraries, which target specific use cases:

  • ModbusSerialMaster — communicate with Modbus devices via the Modbus-RS485 and Modbus-RS232 protocols.
  • ModbusTCPMaster — communicate with Modbus devices via TCP/IP.

We recommend you work with one of these libraries unless your use case very specifically needs to perform PDU operations not provided by them.

You can view the library’s source code on GitHub.

To use this library, add the following statements to the top of your device code:

#require "CRC16.class.nut:1.0.0"
#require "ModbusRTU.device.lib.nut:1.0.2"

ModbusRTU Class Usage

This is the main library class. All methods and variables inside this class are static.

createReadPDU(targetType, startingAddress, quantity)

This method creates a PDU for readData operations. It takes the following parameters:

Parameter Data Type Required Default Value Description
targetType Constant Yes N/A Refer to the Target Type table, below
startingAddress Integer Yes N/A The address from which it begins reading values
quantity Integer Yes N/A The number of consecutive addresses the values are read from

 

Type Value
Coil MODBUSRTU_TARGET_TYPE.COIL
Discrete Input MODBUSRTU_TARGET_TYPE.DISCRETE_INPUT
Input Register MODBUSRTU_TARGET_TYPE.INPUT_REGISTER
Holding Register MODBUSRTU_TARGET_TYPE.HOLDING_REGISTER

Example

local targetType = MODBUSRTU_TARGET_TYPE.COIL;
local startingAddress = 0x01;
local quantity = 1;

local pdu = ModbusRTU.createReadPDU(targetType, startingAddress, quantity);
server.log(pdu);

createWritePDU(targetType, startingAddress, quantity, values)

This method creates a PDU for writeData operations. It takes the following parameters:

Parameter Data Type Required Default Value Description
targetType Constant Yes N/A Refer to the Target Type table, above
startingAddress Integer Yes N/A The address from which it begins writing values
quantity Integer Yes N/A The number of consecutive addresses the values are written into
values Integer, array ([integer, boolean]), boolean, blob Yes N/A The values written into Coils or Registers

 

Type Value
Coil MODBUSRTU_TARGET_TYPE.COIL
Holding Register MODBUSRTU_TARGET_TYPE.HOLDING_REGISTER

Example

local targetType = MODBUSRTU_TARGET_TYPE.COIL;
local startingAddress = 0x01;
local values = false;
local quantity = 1;

local pdu = ModbusRTU.createWritePDU(targetType, startingAddress, quantity, values);
server.log(pdu);

createReadExceptionStatusPDU()

This method creates a PDU for readExceptionStatus operations. It takes no parameters.

Example

local pdu = ModbusRTU.createReadExceptionStatusPDU();
server.log(pdu);

createDiagnosticsPDU(subFunctionCode, data)

This method creates a PDU for diagnostics operations. It takes the following parameters:

Parameter Data Type Required Default Value Description
subFunctionCode Constant Yes N/A Refer to the ‘Sub-function Code’ table, below
data Blob Yes N/A The data field required by Modbus request

 

Sub-function Code Value (Hex)
MODBUSRTU_SUB_FUNCTION_CODE.RETURN_QUERY_DATA 0x0000
MODBUSRTU_SUB_FUNCTION_CODE.RESTART_COMMUNICATION_OPTION 0x0001
MODBUSRTU_SUB_FUNCTION_CODE.RETURN_DIAGNOSTICS_REGISTER 0x0002
MODBUSRTU_SUB_FUNCTION_CODE.CHANGE_ASCII_INPUT_DELIMITER 0x0003
MODBUSRTU_SUB_FUNCTION_CODE.FORCE_LISTEN_ONLY_MODE 0x0004
MODBUSRTU_SUB_FUNCTION_CODE.CLEAR_COUNTERS_AND_DIAGNOSTICS_REGISTER 0x000A
MODBUSRTU_SUB_FUNCTION_CODE.RETURN_BUS_MESSAGE_COUNT 0x000B
MODBUSRTU_SUB_FUNCTION_CODE.RETURN_BUS_COMMUNICATION_ERROR_COUNT 0x000C
MODBUSRTU_SUB_FUNCTION_CODE.RETURN_BUS_EXCEPTION_ERROR_COUNT 0x000D
MODBUSRTU_SUB_FUNCTION_CODE.RETURN_SLAVE_MESSAGE_COUNT 0x000E
MODBUSRTU_SUB_FUNCTION_CODE.RETURN_SLAVE_NO_RESPONSE_COUNT 0x000F
MODBUSRTU_SUB_FUNCTION_CODE.RETURN_SLAVE_NAK_COUNT 0x0010
MODBUSRTU_SUB_FUNCTION_CODE.RETURN_SLAVE_BUSY_COUNT 0x0011
MODBUSRTU_SUB_FUNCTION_CODE.RETURN_BUS_CHARACTER_OVERRUN_COUNT 0x0012
MODBUSRTU_SUB_FUNCTION_CODE.CLEAR_OVERRUN_COUNTER_AND_FLAG 0x0014

Example

local data = blob();
data.writen(swap2(0xFF00),'w');
local pdu = ModbusRTU.createDiagnosticsPDU(MODBUSRTU_SUB_FUNCTION_CODE.RETURN_QUERY_DATA, data);
server.log(pdu);

createReportSlaveIdPDU()

This method creates a PDU for reportSlaveId operations. It has no parameters.

Example

local pdu = ModbusRTU.createReportSlaveIdPDU();
server.log(pdu);

createMaskWriteRegisterPDU(referenceAddress, AND_Mask, OR_Mask)

This method creates a PDU for maskWriteRegister operations. It takes the following parameters:

Parameter Data Type Required Default Value Description
referenceAddress Integer Yes N/A The address of the holding register the value is written into
AND_mask Integer Yes N/A The AND mask
OR_mask Integer Yes N/A The OR mask

Example

local referenceAddress = 0x0A;
local AND_mask = 0xFFFF;
local OR_mask = 0x0000;
local pdu = ModbusRTU.createMaskWriteRegisterPDU(referenceAddress, AND_mask, OR_mask);
server.log(pdu);

createReadWriteMultipleRegistersPDU(readingStartAddress, readQuantity, writeStartAddress, writeQuantity, writeValue)

This method creates a PDU for readWriteMultipleRegisters operations. It takes the following parameters:

Parameter Data Type Required Default Value Description
readingStartAddress Integer Yes N/A The address from which it begins reading values
readQuantity Integer Yes N/A The number of consecutive addresses values are read from
writeStartAddress Integer Yes N/A The address from which it begins writing values
writeQuantity Integer Yes N/A The number of consecutive addresses values are written into
writeValue Blob Yes N/A The value written into the holding register

Example

local readingStartAddress = 0x01;
local readQuantity = 1;
local writeStartAddress = 0x0A;
local writeValue = blob();
writeValue.writen(swap2(28),'w');
local writeQuantity = writeValue.len() / 2 ;
local pdu = ModbusRTU.createReadWriteMultipleRegistersPDU(readingStartAddress, readQuantity, writeStartAddress, writeQuantity, writeValue);
server.log(pdu);

createReadDeviceIdentificationPDU(readDeviceIdCode, objectId)

This method creates a PDU for readDeviceIdentificationPDU operations. It takes the following parameters:

Key Data Type Required Default Value Description
readDeviceIdCode Constant Yes N/A Refer to the ‘Read Device ID Code’ table, below
objectId Constant Yes N/A Refer to the ‘Object ID’ Code table, below

 

Read Device ID Code Description
MODBUSRTU_READ_DEVICE_CODE.BASIC Get the basic device identification (stream access)
MODBUSRTU_READ_DEVICE_CODE.REGULAR Get the regular device identification (stream access)
MODBUSRTU_READ_DEVICE_CODE.EXTENDED Get the extended device identification (stream access)
MODBUSRTU_READ_DEVICE_CODE.SPECIFIC Get one specific identification object (individual access)

 

Object ID Category
MODBUSRTU_OBJECT_ID.VENDOR_NAME Basic
MODBUSRTU_OBJECT_ID.PRODUCT_CODE Basic
MODBUSRTU_OBJECT_ID.MAJOR_MINOR_REVISION Basic
MODBUSRTU_OBJECT_ID.VENDOR_URL Regular
MODBUSRTU_OBJECT_ID.PRODUCT_NAME Regular
MODBUSRTU_OBJECT_ID.MODEL_NAME Regular
MODBUSRTU_OBJECT_ID.USER_APPLICATION_NAME Regular

Example

local readDeviceIdCode = MODBUSRTU_READ_DEVICE_CODE.BASIC;
local objectId = MODBUSRTU_OBJECT_ID.VENDOR_NAME;
local pdu = ModbusRTU.createReadDeviceIdentificationPDU(readDeviceIdCode, objectId);
server.log(pdu);

parse(params)

This method parses the Modbus PDU and returns the result. The value passed into the parameter, params, is a table composed of the following keys:

Key Data Type Required Default Value Description
quantity Integer No N/A The number of coils/register to read from or write into
PDU Blob Yes N/A The PDU to be parsed
expectedResType Constant Yes N/A The expected response type
expectedResLen Integer Yes N/A The expected response length

Example

local quantity = 1;
local pdu = blob();
local expectedResType = ModbusRTU.FUNCTION_CODES.readCoils.fcode;
local expectedResLen = ModbusRTU.FUNCTION_CODES.readCoils.resLen(quantity);
local result = parse({
    "quantity"        : quantity,
    "PDU"             : pdu,
    "expectedResLen"  : expectedResLen,
    "expectedResType" : expectedResType
 });

License

The ModbusRTU library is licensed under the MIT License.

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