Modbus network mode

Modbus protocol introduction MODBUS was invented by Modicon (now a brand of Schneider Electric Company) in 1979. It is the first real bus protocol used in industrial field in the world.

 

Product description  

1. Technical index of IDCB remote I / O MODBUS network

. number of connected main stations: 1-2

. the definition range of front-end address : 1-255

. main station type: DCS, industrial computer, compatible computer, notebook, portable computer.

. number of connected slave stations (i.e. number of front ends that can be connected in a network): ≤ 50

. network topology: linear bus, with Terminator (matching resistance) at both ends when the distance is long, with short cut ≤ 0.3m, no branch

. transmission medium: shielded twisted pair

. communication rate: 9.6K, 19.2k, 38.4k, 115.2k can be set.

. basic communication distance: 1.2km

. network communication mode: half duplex asynchronous communication

. communication physical layer: RS485

. network isolation: 600V

. data format: 1 start bit, 8 data bits, 1 stop bit, no parity

. communication protocol: Modbus protocol RTU mode

. communication verification: CRC cyclic redundancy verification

 

2. Main models of Modbus network products

Only two models which are widely used in this series of products are introduced below. For other I / O configurations, please refer to the introduction of basic models

2.1 IDCB-4E / 24A  DC analog measurement front end

Number of input channels: 20 / 24 three terminal differential input

Channel isolation voltage: 400V (peak to peak)

Network isolation voltage: 600V

Channel switch: imported photoelectric switch

Measurement object: DC voltage, DC current

Thermocouple: K, e, s, t, EA-2

Thermal resistance: PT100, cu50, cu100, BA1, Ba2, g53

Thermocouple cold end compensation mode: automatic (ambient temperature) compensation, constant temperature compensation, other channel compensation

Thermal resistance measurement method: 3 / 2 wire system

Resolution: 16 bits

Measurement accuracy: class 0.1

Channel sampling time: 30 points / S

Common mode rejection ratio: 120dB

Serial mode rejection ratio: 70dB

 

Main functions:

Table checking operation of thermal resistance and thermocouple

Linear and square operation of current and voltage signals

System self-test 

Accuracy self calibration

Watchdog circuit

Power failure protection in working mode

LED status indication

Field display and configuration (/ Y type)

Ambient temperature: - 20 ℃ ~ 75 ℃

Relative humidity ≤ 95%

Power supply: 100-265VAC

Protection grade: IP56 ~ IP66

 

2.2. Main technical indicators of the front-end of DC analog measurement of IDCB-2a / DR

. number of input channels: 32 passive dry contact switch input

. network isolation voltage: 600V

. communication interface: redundant RS485

. communication protocol: Modbus RTU

. communication rate: 9.6K, 19.2k, 38.4k, 115.2k can be set.

. network redundancy mode: dual RS485 interface.

. basic communication distance: 1.2km

. input mode: photoelectric isolation

. measurement object: switching value: passive dry contact

. main function: switch state measurement system self check

LED status indication

Field configurator configuration

Ambient temperature: - 20 ℃ ~ 75 ℃

Relative humidity ≤ 95%

Power supply: 100-265VAC

Protection grade: IP56 ~ IP66

 

3. Interface between IDCB series MODBUS network and DCS system

3.1 interface mode:

Communication mode: half duplex asynchronous communication

Interface model: RS485 / 232 interface

Communication protocol: Modbus RTU

Master-slave relationship: DCS is the master station, remote I / O front-end is the slave station

Communication rate: 9.6kbps to 115.2kbps optional

Data format: start bit 1 / data bit 8 / stop bit 1 / no parity

3.2 command format

The format of all commands for reading front-end data is as follows (1B in brackets represents 1 byte):

Device address (1b), function code (1b), high byte of data start address (1b), low byte of data start address (1b), high byte of data to be read (1b), low byte of data to be read (1b), low byte of CRC check value (1b), high byte of CRC check value (1b).

The data format returned by all front-end read commands is as follows:

Device address (1b), function code (1b), number of bytes (1b) (twice the number of data to be read, i.e. two bytes for each data), data (the number of bytes is equal to the third data value, i.e. the number of bytes), CRC check (low, 1b), CRC check (high, 1b).