Class: ModbusServer 

Nodbus-Plus v1.0 Documentation

Table of Contents

Class: ModbusServer

The ModbusServer class is an EventEmitter that provides basic functionalities to handle Modbus Protocol Data Units (PDU).

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

Creating a ModbusServer Instance 

new ModbusServer([options])

options <object>: Configuration object with following properties:
- inputs <number>: The cuantity of inputs that the server will have. It’s an integer between 0 and 65535. If a value of 0 is entered, then the inputs will share the same Buffer as the inputs registers. Default value is 2048.
- coils <number>: The cuantity of coils that the server will have. It’s an integer between 0 and 65535. If a value of 0 is entered, then the coils will share the same Buffer as holding registers. Default value is 2048.
- holdingRegisters <number>: The cuantity of holding registers that the server will have. It’s an integer between 1 and 65535. Default value is 2048.
- inputRegisters <number>: The cuantity of input registers that the server will have. It’s an integer between 1 and 65535. Default value is 2048.
Returns: <ModbusServer>

Constructor for new ModbusServer instance.

const ModbusServer = require('nodbus-plus').ModbusServer;
let modbusServer = new ModbusServer({inputs: 1024, coils: 512}); //new server with 1024 inputs, 512 coils and 2048 holding and inputs registers

ModbusServer’s Events 

Event: ‘error’

e <Error>: The error object.

Emitted when a error occurs.

Event: ‘write-coils’

startCoil <number> Indicate in wich coil start the new value.
cuantityOfCoils <number>: amound of coils modificated

Emitted after change a coil value due to a client write coil request.

Event: ‘write-registers’

startRegister <number> Indicate in wich register start the new value.
cuantityOfRegister <number>: amound of register modificated.

Emitted after change a holding register value due to a client write register request.

This property stores the Modbus functions codes supported by the server. It’s a map composed of an integer number with the Modbus function code as the key and the name of the method that will be invoked to resolve that code as the value.

//Example of how to add new custom modbus function code handle function
class ModbusServerExtended extends ModbusServer{
      constructor(mbServerCfg){
            super(mbServerCfg)
            //adding the new function code and the name of handler
            this._internalFunctionCode.set(68, 'customService68');
      }
      //New method to handle function code 68. receive a buffer with pdu data as argument.
      customService68(pduReqData){
            let resp = Buffer.alloc(2);
            resp[0] = 68;
            resp[1] = pduReqData[0];
            return resp
      }
}

Atribute: modbusServer.supportedFunctionCode 

<iterator>

This is a getter that return an iterator object trhough modbusServer._internalFunctionCode keys. It’s the same that call modbusServer._internalFunctionCode.keys().

//Example of getting all suported function code.
for(const functionCode of modbusServer.supportedFunctionCode){
   console.log(functionCode)
}

Atribute: modbusServer.holdingRegisters 

<Buffer>

This property is a Buffer that store the servers’ holding registers. The Modbus protocol specifies the order in which bytes are sent and receive. Modbus Plus uses a big-endian encoding to send the content of 16-bit registers. This means that byte[0] of the register will be considered the MSB and byte[1] the LSB.

Each register starts at the even byte of the buffer.Therefore, register 0 starts at byte 0 and occupies bytes 0 and 1, register 1 starts at byte 2 and occupies bytes 2 and 3, and so on.

To read or write values in the registers, you can use the buffer’s methods (see Node.js documentation), but it is recommended to use the getWordFromBuffer method and the setWordtoBuffer method.

Atribute: modbusServer.inputRegisters 

<Buffer>

This property is a Buffer that store the servers’ input registers. The Modbus protocol specifies the order in which bytes are sent and receive. Modbus Plus uses a big-endian encoding to send the content of 16-bit registers. This means that byte[0] of the register will be considered the MSB and byte[1] the LSB.

Each register starts at the even byte of the buffer.Therefore, register 0 starts at byte 0 and occupies bytes 0 and 1, register 1 starts at byte 2 and occupies bytes 2 and 3, and so on.

To read or write values in the registers, you can use the buffer’s methods (see Node.js documentation), but it is recommended to use the getWordFromBuffer method and the setWordtoBuffer method.

Atribute: modbusServer.inputs 

<Buffer>

This property is a Buffer that store the servers’ digital inputs. The byte 0 store the inputs 0 to 7, byte 1 store inputs 8-15 and so on.

To read and write digital values to the buffer, the modbus server provides the methods getBoolFromBuffer and setBooltoBuffer method.

Atribute: modbusServer.coils 

<Buffer>

This property is a Buffer that store the servers’ digital coils. The byte 0 store the coils 0 to 7, byte 1 store coils 8-15 and so on.

To read and write digital values to the buffer, the modbus server provides the methods getBoolFromBuffer and setBooltoBuffer method.

ModbusServer’s Methods 

Method: modbusServer.processReqPdu(reqPduBuffer)

reqPduBuffer <Buffer>: A buffer containind the data part from request pdu.
Returns <Buffer>: Complete response pdu’s buffer.

This is the server’s main function. Receive a request pdu buffer, and return a response pdu that can be a normal response or exception response.

Method: modbusServer.makeExceptionResPdu(mbFunctionCode, exceptionCode)

mbFunctionCode <number>: The function code that cause the exception.
exceptionCode <number>: See available exception code on Event: ‘exception’
Returns <Buffer>: Exception response pdu

This functions create a exception response pdu by add 0x80 to function code and appending the exception code.

Method: modbusServer.readCoilsService(pduReqData)

pduReqData <Buffer>: buffer containig the pdu’s data.
Return <Buffer>: buffer with response pdu.

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

This method execute the read coil status indication on the server. This method is not intended to be called directly, but instead through the method processReqPdu when function code 01 is received.

Method: modbusServer.readDiscreteInputsService(pduReqData)

pduReqData <Buffer>: buffer containig the pdu’s data.
Return <Buffer>: buffer with response pdu.

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

This method execute the read digital input status indication on the server. This method is not intended to be called directly, but instead through the method processReqPdu when function code 02 is received.

Method: modbusServer.readHoldingRegistersService(pduReqData)

pduReqData <Buffer>: buffer containig the pdu’s data.
Return <Buffer>: buffer with response pdu.

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

This method execute the read holding registers indication on the server. This method is not intended to be called directly, but instead through the method processReqPdu when function code 03 is received.

Method: modbusServer.readInputRegistersService(pduReqData)

pduReqData <Buffer>: buffer containig the pdu’s data.
Return <Buffer>: buffer with response pdu.

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

This method execute the read input registers indication on the server. This method is not intended to be called directly, but instead through the method processReqPdu when function code 04 is received.

Method: modbusServer.writeSingleCoilService(pduReqData)

pduReqData <Buffer>: buffer containig the pdu’s data.
Return <Buffer>: buffer with response pdu.

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

This method execute the write single coil indication on the server. This method is not intended to be called directly, but instead through the method processReqPdu when function code 05 is received.

Method: modbusServer.writeSingleRegisterService(pduReqData)

pduReqData <Buffer>: buffer containig the pdu’s data.
Return <Buffer>: buffer with response pdu.

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

This method execute the write single register indication on the server. This method is not intended to be called directly, but instead through the method processReqPdu when function code 06 is received.

Method: modbusServer.writeMultipleCoilsService(pduReqData)

pduReqData <Buffer>: buffer containig the pdu’s data.
Return <Buffer>: buffer with response pdu.

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

This method execute the write multiple coils indication on the server. This method is not intended to be called directly, but instead through the method processReqPdu when function code 15 is received.

Method: modbusServer.writeMultipleRegistersService(pduReqData)

pduReqData <Buffer>: buffer containig the pdu’s data.
Return <Buffer>: buffer with response pdu.

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

This method execute the write multiple registers indication on the server. This method is not intended to be called directly, but instead through the method processReqPdu when function code 16 is received.

Method: modbusServer.maskWriteRegisterService(pduReqData)

pduReqData <Buffer>: buffer containig the pdu’s data.
Return <Buffer>: buffer with response pdu.

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

This method execute the mask register indication on the server. This method is not intended to be called directly, but instead through the method processReqPdu when function code 22 is received.

Method: modbusServer.readWriteMultipleRegistersService(pduReqData)

pduReqData <Buffer>: buffer containig the pdu’s data.
Return <Buffer>: buffer with response pdu.

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

This method execute the read and write multiple registers indication on the server. This method is not intended to be called directly, but instead through the method processReqPdu when function code 23 is received.

Method: modbusServer.getBoolFromBuffer(targetBuffer, [offset])

targetBuffer <Buffer>: Buffer with the objetive boolean value to read.
offset <number>: A number with value’s offset inside the buffer.
Return <boolean>: value.

This method read a boolean value inside a buffer. The buffer’s first byte store the 0-7 boolean values’s offset. Example:

modbusServer.inputs[0] = 0x44  //first byte 0100 0100
modbusServer.coils[1] =  0x55 //second byte 0101 0101

modbusServer.getBoolFromBuffer(modbusServer.inputs, 6) //return 1
modbusServer.getBoolFromBuffer(modbusServer.coils, 5) //return 0

Method: modbusServer.setBoolToBuffer(value, targetBuffer, [offset])

value <boolean>: Value to write.
targetBuffer <Buffer>: Buffer with the objetive boolean value to write.
offset <number>: A number with value’s offset inside the buffer.

This method write a boolean value inside a buffer. The buffer’s first byte store the 0-7 boolean values’s offset. Example:

modbusServer.getBoolFromBuffer(true, modbusServer.coils, 5)
console.log(modbusServer.coils[1])  //now second byte is 0x75 (0111 0101)

Method: modbusServer.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

modbusServer.holdingRegisters[0] = 0x11;
modbusServer.holdingRegisters[1] = 0x22;
modbusServer.holdingRegisters[2] = 0x33;
modbusServer.holdingRegisters[3] = 0x44;

modbusServer.holdingRegisters.readUInt16BE(0)                           //returns 0x1122
modbusServer.holdingRegisters.readUInt16BE(1)                           //returns 0x2233
modbusServer.getWordFromBuffer(modbusServer.holdingRegisters, 0)        //returns Buffer:[0x11, 0x22]
modbusServer.getWordFromBuffer(modbusServer.holdingRegisters, 1)        //returns Buffer:[0x33, 0x44]

Method: modbusServer.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. Example:

let realValue = Buffer.alloc(4);
realValue.writeFloatBE(3.14);
let register1 = realValue.subarray(0, 2);
let register2 = realValue.subarray(2, 4);

//writing pi value in bytes 2, 3, 4, 5
modbusServer.setWordToBuffer(register1, modbusServer.holdingRegisters, 1);
modbusServer.setWordToBuffer(register2, modbusServer.holdingRegisters, 2);

//instead this write pi value in bytes 1, 2, 3, 4
modbusServer.holdingRegisters.writefloatBE(3.14, 1) //alignment problem

Code	Name	Meaning
01	ILLEGAL FUNCTION	The function code received in the query is not an allowable action for the server.
02	ILLEGAL DATA ADDRESS	The data address received in the query is not an allowable address for the server.
03	ILLEGAL DATA VALUE	A value contained in the query data field is not an allowable value for server
04	SLAVE DEVICE FAILURE	An unrecoverable error occurred while the server was attempting to perform the requested action.
05	ACKNOWLEDGE	The server (or slave) has accepted the request and is processing it, but a long duration of time will be required to do so. This response is returned to prevent a timeout error from occurringin the client (or master).
06	SLAVE DEVICE BUSY	Specialized use in conjunction with programming commands. The server (or slave) is engaged in processing a long–duration program command.
08	MEMORY PARITY ERROR	Specialized use in conjunction with function codes 20 and 21 and reference type 6, to indicate that the extended file area failed to pass a consistency check.
0A	GATEWAY PATH UNAVAILABLE	Specialized use in conjunction with gateways, indicates that the gateway was unable to allocate an internal communication path from the input port to the output port for processing the request. Usually means that the gateway is misconfigured or overloaded.
0B	GATEWAY TARGET DEVICE FAILED TO RESPOND	Specialized use in conjunction with gateways, indicates that no response was obtained from the target device. Usually means that the device is not present on the network.