Class: ModbusClient 

Table of Contents

Class: ModbusClient
- Creating a ModbusClient Instance
  - new ModbusClient()
- ModbusClient’s Methods

Nodbus-Plus v1.0 Documentation

This class is an EventEmitter. It provides the basic functionalities to make Modbus Protocol Data Units (PDU).

../_images/modbus_pdu.png — *Modbus Protocol Data Unis*

Creating a ModbusClient Instance 

new ModbusClient()

Returns: <ModbusClient>

Constructor for new ModbusClient instance.

const ModbusClient = require('nodbus-plus').ModbusClient;
let modbusClient = new ModbusClient();

ModbusClient’s Methods 

Method: modbusClient.readCoilStatusPdu([startCoil],[coilQuantity])

startCoil <number>: First coil to read, starting at address 0. Default value is 0
coilQuantity <number>: Number of coils to read. Default value is 1
Return <Buffer>: buffer with req pdu.

../_images/01-readcoils.png — *Modbus Read Coils Request and Response*

This method create the read coil status request pdu. Function code 01.

Method: modbusClient.readInputStatusPdu([startInput],[inputQuantity])

startInput <number>: First input to read, starting at address 0. Default value is 0
inputQuantity <number>: Number of inputs to read. Default value is 1
Return <Buffer>: buffer with req pdu.

../_images/02-readinputs.png — *Modbus Read Inputs Request and Response*

This method create the read input status request pdu. Function code 02.

Method: modbusClient.readHoldingRegistersPdu([startRegister],[registerQuantity])

startRegister <number>: First register to read, starting at address 0. Default value is 0
registerQuantity <number>: Number of registers to read. Default value is 1
Return <Buffer>: buffer with req pdu.

../_images/03-readholding.png — *Modbus Read Holding Registers Request and Response*

This method create the read holding register request pdu. Function code 03.

Method: modbusClient.readInputRegistersPdu([startRegister],[registerQuantity])

startRegister <number>: First register to read, starting at address 0. Default value is 0
registerQuantity <number>: Number of registers to read. Default value is 1
Return <Buffer>: buffer with req pdu.

../_images/04-readinputsreg.png — *Modbus Read Inputs Registers Request and Response*

This method create the read inputs register request pdu. Function code 04.

Method: modbusClient.forceSingleCoilPdu(value, [startCoil])

value <Buffer>: Two bytes length buffer. Valid values are [0x00, 0x00] for false and [0xFF, 0x00] for true.
startCoil <number>: Coil to be writed. Default value is 0.
Return <Buffer>: buffer with req pdu.

../_images/05-writecoil.png — *Modbus Write Single Coil Request and Response*

This method create the force single coil request pdu. Function code 05. If value is not a Buffer throw a TypeError and if value’s length is diferent than 2 throw a RangeError.

Method: modbusClient.presetSingleRegisterPdu(value, [startRegister])

value <Buffer>: Two bytes length buffer.
startRegister <number>: Register’s address to be writed. Default value is 0.
Return <Buffer>: buffer with req pdu.

../_images/06-writeregister.png — *Modbus Write Single holding Register Request and Response*

This method create the preset single register request pdu. Function code 06. If value is not a Buffer throw a TypeError and if value’s length is diferent than 2 throw a RangeError.

Method: modbusClient.forceMultipleCoilsPdu(values, startCoil, coilQuantity)

values <Buffer>: Buffer with coils values.
startCoil <number>: First Coil starting address.
coilQuantity <number>: Number of coils to write.
Return <Buffer>: buffer with req pdu.

../_images/15-writecoil.png — *Modbus Write Multiple Coils Request and Response*

This method create the force multiples coils request pdu. Function code 15. If values is not a Buffer throw a TypeError and if value’s length is higher than 246 throw a RangeError.

Method: modbusClient.presetMultipleRegistersPdu(values, startRegister, [registerQuantity])

values <Buffer>: Buffer with registers values.
startRegister <number>: First register starting address.
registerQuantity <number>: Number of registers to write. Default value is values.length/2.
Return <Buffer>: buffer with req pdu.

../_images/16.png — *Modbus Write Multiple Registers Request and Response*

This method create the preset multiples registers request pdu. Function code 16. If values is not a Buffer throw a TypeError and if value’s length is higher than 246 throw a RangeError.

Method: modbusClient.maskHoldingRegisterPdu(values, [startRegister])

values <Buffer>: Buffer with registers values.
startRegister <number>: Register address to modify.
Return <Buffer>: buffer with req pdu.

../_images/22-mask.png — *Modbus Mask Register Request and Response*

This method create the mask holding register request pdu. Function code 22. If values is not a Buffer throw a TypeError and if value’s length is diferent than 4 throw a RangeError.

Method: modbusClient.readWriteMultipleRegistersPdu(values, readStartingAddress, quantitytoRead, writeStartingAddress, quantityToWrite)

values <Buffer>: Buffer with registers values to write.
readStartingAddress <number>: First register’s address to read.
quantitytoRead <number>: Number of register to read.
writeStartingAddress <number>: First register’s address to write.
quantityToWrite <number>: Number of registers to write.
Return <Buffer>: buffer with req pdu.

../_images/23.png — *Modbus Read and Write Multiple Registers Request and Response*

This method create the read and write holding register request pdu. Function code 23. If values is not a Buffer throw a TypeError and if value’s length is greater than 243 throw a RangeError.

Method: modbusClient.boolToBuffer(value)

value <boolean>
Return <Buffer>: Two bytes length Buffer.

This is a utitlity method. It returns a buffer with a boolean value encoded for use on forceSingleCoilPdu function as value argument. Example:

let value = modbusClient.boolToBuffer(false);
console.log(value); //Buffer:[0x00, 0x00]
value = modbusClient.boolToBuffer(true);
console.log(value); //Buffer:[0xFF, 0x00]

Method: modbusClient.getMaskRegisterBuffer(value)

value <Array>: An 16 numbers length array indicating how to mask the register.
Return <Buffer>: Four bytes length Buffer.

This is a utility method that return a four-byte length buffer with the AND_MASK and OR_MASK values encoded for use in the maskHoldingRegisterPdu function as the value argument.

The value argument is a 16-number array, with each number representing the position of one bit inside the register. If the number is 1, then the corresponding bit will be set to 1. If the number is 0, then the corresponding bit will be set to 0. If the number is different from 0 or 1, then the corresponding bit will remain unchanged. For example:

let value = [-1, 0, 1, -1, -1, -1, 0, 0, 1, -1, -1, -1, -1, -1, 1, 1];
maskBuffer = modbusClient.getMaskRegisterBuffer(value);

//masks
let andMask =  maskBuffer.readUInt16BE(0);
let orMask =   maskBuffer.readUInt16BE(2);

let testRegister = Buffer.from([0x9A, 0xFB]);
console.log(testRegister)
let currentContent = testRegister.readUInt16BE(0);
let finalResult = (currentContent & andMask) | (orMask & (~andMask)); //Modbus Spec

let finalRegister = Buffer.alloc(2);
finalRegister.writeUInt16BE(finalResult, 0);
console.log(finalRegister)

//Output
//<Buffer 9a fb>
//<Buffer db 3d>

Method: modbusClient.boolsToBuffer(value)

value <Array>: A boolean array.
Return <Buffer>: a buffer with binary representation of boolean array.

This is a utility method that return a buffer from a boolean array for modbus function code 15.

The value argument is an array of boolean with values to be force to coils. For example:

let values = [0, 1, 0, 0, 0, 0, 1, 1, 0, 0, 1]; //at 0 index stat LSB Byte
valBuffer = modbusClient.boolsToBuffer(values);

//result valBuffer [0xC2 0x04]
// calling force multiples colis
let pdu = modbusClient.forceMultipleCoilsPdu(valBuffer, 10, values.length)  //calling force multiples coils at coil 10 and 11 coils to force

Method: modbusClient.getWordFromBuffer(targetBuffer, [offset])

targetBuffer <Buffer>: Buffer with the objetive 16 bits register to read.
offset <number>: A number with register’s offset inside the buffer.
Return <Buffer>: A two bytes length buffer.

This method read two bytes from target buffer with 16 bits align. Offset 0 get bytes 0 and 1, offset 4 gets bytes 8 and 9

Method: modbusClient.setWordToBuffer(value, targetBuffer, [offset])

value <Buffer>: two bytes length buffer.
targetBuffer <Buffer>: Buffer with the objetive 16 bits register to write.
offset <number>: A number with register’s offset inside the buffer.

This method write a 16 bits register inside a buffer. The offset is 16 bits aligned.