Data read and written by this module are exchanged as buffers, i.e. either strings or lists of strings. String(s) contain data as 8-bits bytes; endianness depends on what is expected/returned by the slave device. To encode such strings, it is suggested to rely on pack, the binary string packing library.

Type modbus

Type modbusdev

Type modbus

Field(s)

modbus.new(port, cfg, mode)

Parameters

• port : the serial port on which modbus operates. As for serial.open, it can be given as a port number, or as a file path (POSIX operating systems only). Defaults to port 0, i.e. "/dev/ttyS0" on POSIX.
• cfg : a optional serial device config table, see serial.config for details on accepted fields.
  Same default values as in serial.open. An additional field 'timeout' can be put in cfg as an integer to configure modbus timeout in seconds.
• mode : serial mode as a string: "ASCII" or "RTU".
  Defaults to "RTU".

Return values

1. #modbusdev: the new #modbusdev on success
2. nil + error message

Type modbusdev

Field(s)

modbusdev:close()

Return value

nothing.

modbusdev:customRequest(req, sid, payload)

Parameters

• req : number identifying the custom function
• sid : slave id
• payload : optional data bytes to send, as a string

Return values

1. the response as a buffer. Beware, it contains some error-checking bytes as a suffix.
2. nil + error message.

modbusdev:readCoils(sid, address, length)

Parameters

• sid : number defining the slave id to send the request to
• address : starting address
• length : number of coils

Return values

1. read coil values in a buffer
2. nil + error message

modbusdev:readDiscreteInputs(sid, address, length)

Parameters

• sid : number defining the slave id to send the request to
• address : starting address
• length : number of inputs

Return values

1. read discrete inputs contant in a buffer
2. nil + error message

modbusdev:readHoldingRegisters(sid, address, length)

Parameters

• sid : number defining the slave id to send the request to
• address : starting address
• length : number of registers

Return values

1. read holding registers content in a buffer
2. nil + error message

modbusdev:readInputRegisters(sid, address, length)

Parameters

• sid : number defining the slave id to send the request to
• address : starting address
• length : number of inputs

Return values

1. read input registers content in a buffer
2. nil + error message

modbusdev:writeMultipleCoils(sid, address, values)

Parameters

• sid : number defining the slave id to send the request to
• address : starting address
• values : values to write, as a bytes buffer (8 coil per byte)

Return values

1. "ok"
2. nil + error message

Usage:

  m = modbus.new('/dev/ttyS0')
  res,err=m:writeMultipleCoils(1, 0, 8, string.pack('x8', 
  true,false,false,true,false,true,false,true)

modbusdev:writeMultipleRegisters(sid, address, values)

Parameters

• sid : number defining the slave id to send the request to
• address : starting address
• values :

Return values

1. "ok"
2. nil + error message

modbusdev:writeSingleCoil(sid, address, value)

Parameters

• sid : number defining the slave id to send the request to
• address : address of the coil
• value : Boolean to write in the coil

Return values

1. "ok"
2. nil + error message

modbusdev:writeSingleRegister(sid, address, value)

Parameters

• sid : number defining the slave id to send the request to
• address : address of the coil
• value : Short integer ([0..0xFFFF]) to write in the register

Return values

1. "ok"
2. nil + error message