Questions tagged [modbus]

Modbus is a serial communications protocol commonly used by PLCs (programmable logic controllers) and other industrial control systems. While originally based on traditional serial communications many extensions exist to allow communication over other networks such as TCP/IP.

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30 views

Isolating an existing RS485 tranceiver

I have a heat pump with an RS485 interface. Via a simple RS485 chip. No isolated interface, no isolated power supply to it. I want to add a power meter at 400+ meter to it and want to properly isolate ...
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3k views

Difficulty in reading values Modbus application in Python(Pyserial) On Energy meter (EM6400)

I am trying to read energy meter(EM6400) voltage values in python by using modbus protocol. But there are issues when I am reading values. I think the issue is with,...
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57 views

modbus register address vs. register number; absolute address vs. offset

I'm developing a Modbus Slave device, and I'm having a little trouble understanding how to name/address my registers. Let's say I have 2 Discrete Inputs and 2 Input registers. From Modicon standard, ...
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26 views

Is there any limit of how many commands it can be sent to modbus controller relay?

I have modbus controlled relay hooked to raspberry pi (using node-red) which depending on the condition sends "on" or "off" command every 5 seconds (24h/365d). I know that relays in general have its ...
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1answer
63 views

Exactly what is an RTU (Remote Terminal Unit)?

I find the Wikipedia article and other sources ambiguous and I find that the definitions are so broad that I don't know how and when to use the term "RTU". I would like to figure this out because I ...
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242 views

Why is there no reponse from ModbusRTU slave using converter?

There are two devices: Eaton EM20/M (~like Northern Design Cube 400) Volison ADM-5850G Connected them like: Trying to get data from EM20/M which supports ...
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146 views

RS232 Modbus to USB wiring

I have a RS232 device that sends data following the modbus protocol. I have problems communicating with it and I think my wiring is wrong. Since most modbus devices work over RS485 it is hard to find ...
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3answers
4k views

Best way to poll for Modbus RTU slaves through a Modbus TCP gateway

I'm using a number of Modbus RTU devices connected to my PC through a Modbus TCP gateway. I'd like to poll all the available slave addresses to check which ones are available through my program. What ...
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1answer
38 views

Is there some kind of standard of modbus input register?

As the title said, based on your experience, is there a such thing? If no, which are the most commonly used? for example in a sensor, the input registers as follows: ...
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1answer
339 views

Is there a fieldbus with more than 250 nodes per segment?

We plan to realize a sensor cable several 1000m long which has ideally every 1-2m a magnetic field sensor on a small 10x30mm platine with microprocessor/FPGA for DSP and I2C on chip communication. ...
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42 views

How to increase data through put of RS485 RTU?

I am using 20 sensor nodes, each has mpu9250 i2c, Arduino atmega328 MCU 8MHz and max485 chip in it, each slave has a unique id program. I have Arduino due as Modbus master with max485 chip, I hope ...
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2answers
451 views

Modbus communication between Arduino and Unitronics Jazz2 PLC

I am trying to connect Unitronics Jazz2 PLC (JZ20-R16) and an Arduino Uno using Modbus protocol, over RS232 interface. I've got all the required connectors for both Arduino (TTL-RS232 MAX3232 ...
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96 views

Modbus slave firmware development

I'm developing a simple RS485 based MODBUS slave application on a Microchip PIC micro-controller and need to know best practices to follow on its application side. I can guide myself to get the ...
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445 views

Handling register updates in Modbus systems

I am currently developing a Modbus RTU enabled system that consists of DSP and a microprocessor that handles the Modbus communications. The DSP is sending data to the microprocessor and the ...
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2answers
65 views

Wrong output using SN75HVD10 (RS485)

Designing a modbus master using a STM32F030CCT6 and a SN75HVD10 (3.3V). This is the schematic When i send the following message from the UART: 01 03 00 00 00 06 c5 c8 I see the following scopes (...
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91 views

Modbus RTU over RS232 link

I am designing a convertor board that will interface three device with Modbus RTU RS232 port to a Modbus RTU RS485 network. Slaves supports Modbus RTU, but their ports is different from master port. ...
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73 views

Using CANbus cable for RS-485

I have a question regarding an application with a Modbus RTU network. I would like to know if it's possible to use a CANbus cable instead of an RS-485 cable? Here is a picture of the cable:
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223 views

RS485 Pull-up, Pull-down and single termination resistor

Currently I use for Modbus RTU via RS485 with a shielded cable length of about 5 meters and a bitrate of 19.200 bit/s. Wired are 680 Ohm pull-up (A->+5V) and pull-down (B->GND) resistors near to the ...
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118 views

Why lines A and B are HIGH when idle (rs485)

I'm using modbus RTU over RS485 for a project. I use a RaspberryPI as master and Arduino Pro Mini as slaves. My problem is the following : I plug a sensors on the bus and it returs its value. I plug ...
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296 views

Isolated Modbus (RS485) ground and UART noise

I build an isolated RS485 (Modbus) interface board for a sensor. I use ADM2682 as transceiver and VAQE6W-Q24-S12-D as isolated DC-DC converter. All is managed my a STM32L412. There is 2 interesting ...
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2answers
81 views

Please help me understand modbus communications

I'm just starting out with understanding how modbus works over a rs485 protocol. I've received a communication protocol document from a company that makes solar inverters. I want to send a command ...
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1answer
23 views

Can I use solid state output and relay output with the same components?

So I have a OTB1E0DM9LP Schinder Electric I/O distributed module and I want use it'output to control some 24v electro-valve's and some 24V contactors. The module have 2 Solid-state outputs of 24V 0....
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1answer
58 views

Modbus: Sensors timed out when plugging another sensor

I have created an hydroponic system using modbus to communicate between a RaspberryPI server and plug&play arduino sensors. It works but as soon as I plug a sensor on the bus (physically), all the ...
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46 views

Comunicating with Servo Driver using MODBUS

I am very new to MODBUS and am having trouble connecting to my Servo Motor Driver. I am using a Yun Duan A2 that I purchased from Aliexpress so I could learn about controlling Servo Motors using ...
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59 views

Trouble connecting to temperature controller using MODBUS,RS485 and MATLAB

I am trying to communicate with a temperaturature controller ATR-401 using MATLAB but i am having trouble just connecting to it and reading temperature data.I have set baud rate, data bits, parity and ...
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75 views

Detecting Communication Frames

I am sending data between a master and several slaves with RS485 with bytes (8 bit) messages with a very simple frame inspired on Modbus ASCII: ...
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1answer
604 views

ModBus Timeout Error

Anyone very experienced with Modbus & RS-485??? My task is to control a DC motor using micro PIC24. The motor setting like the angle, delay, start/stop, duty cycle, etc will be sent from PC using ...
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1answer
970 views

Implementing Modbus RTU on a UART device

I have a particle counter HPMA115S0 which has a UART communication interface. I bought a TTL UART to RS485 Converter Module to make it work with a PLC over Modbus RTU protocol. I usually work with ...
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2answers
5k views

RS485 without flow control

Is it possible to send/recieve on RS485 without flow control. I am working on stm32f4 platform with a MAX3160 multiprotocol transceiver(supports RS232 and RS485). The UART lines form STM32F4 are ...
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2k views

How to connect Arduino Uno to an energy meter

How do I connect an energy meter, such as the L&T (Larsen & Toubro) Quasar Intelligent meter (datasheet,) which uses Modbus RTU protocol with RS485 communication to an Arduino Uno? I am ...
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136 views

RS485 registers for calculating total power consumption (kwh)

I have an energy meter (OHIO A2000) which has been now discontinued by the manufacturer. Anyway, I'm following the manual for extracting energy parameters through Modbus RS485 communication. I'm ...
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1answer
1k views

Modbus RTU Implementation to Automate Temperature through PIC Microcontroller

I would like some help about implementing the Modbus RTU communications protocol in an automated system that I am trying to develop. I am confused about how to generally set up my network because I ...
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2answers
8k views

How to connect RS485 Device to computer in Modbus RTU

How to physically connect a voltage monitoring meter device(ND20) with RS485 out, to a computer with Modbus RTU? Should I use a converter to RS232 and connect to serial COM port of computer? Should I ...
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64 views

Looking for data sheet of the following Modbus Sensors #updated

I'm a noob in this area, please ignore any stupidity and lack of info and any help is appreciated and I'm ready to provide all clarification on the post if by chance any info is missing. So i have ...
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1answer
700 views

Detect Modbus RTU start of frame and end of frame

I have implemented Modbus RTU slave in PIC12F1840. My problem is I have not implemented detection of start of message in the slave device. Any guidance on how to implement it?
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71 views

RS485 Functionality difference

I would like to major difference between these ICS. Max 485 Low power Slew rate limited SN75176 :Differential Bus Transceiver ADM 2483 Differential Bus Transceiver ADM 2483 is Isolated type power ...
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1answer
45 views

Interface PID temperature control module using Arduino

I need to interface a modular type temperature controller using Arduino. This is the datasheet of temperature controller The datasheet mentioned like this, My questions are, How can I interface( ...
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1answer
229 views

Mega2560 master on RS485 bus stops communicating with VFD when I add another RS485 module

I’m doing a project where I’m building a control panel for a VFD (inverter/variable frequency drive). I’m connecting the two over half duplex RS485 and use Modbus RTU. As a master I use a Arduino Mega ...
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521 views

Modbus RTU for FPGA

I need to implement Modbus RTU in FPGA (Cyclone 3). Basically I see two ways to do that. The first one is to hardcode it in VHDL and it doesn't seem to be that hard at the begining. But I have looked ...
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2answers
3k views

Difference between Multidrop and daisy chain topology

I want ton know how multidrop differ from daisy chain topology? I am using RS 485 standard and modbus protocol for communication. So which topology is best for my requirement ?
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73 views

A question about “Modbus compatibility”

This is the first time in my life I need to try "Modbus RTU", and as a master I need a RS485 to USB converter which will establish connection to the PC. I have found some converters in the market ...
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1answer
342 views

Read multiple modbus devices information by multiple PLC

The requirement is to read many different Modbus devices by different masters. Since this can't be done through RS485, so the following image is the solution coming to me. My questions are: 1.Is it ...
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1answer
91 views

Suitable 12VDC Communication protocol for untwisted unshielded long-ish wire length?

What would be a reliable and simple tranceiver interface/protocol for board to board serial communication at about 100ft. [9600bps or lower is fine.] The two-lead communication wiring cannot be ...
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49 views

running modbus using cat-5e structured cable plant [duplicate]

We're looking to install some temperature/humidity monitoring in an existing facility. Installing new cabling would be very expensive because of the nature of the plant. Using the existing cat-5e ...
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1answer
66 views

way to display the data from a multi function meter [closed]

I have a multi function meter and i want to collect the data and display it. My idea was to create a api for serial communication?? will the idea work or is there any other better way?? the meter uses ...
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1answer
1k views

Besides address is there other limit on max nodes via RS485 Modbus-RTU

Since there is 1byte for nodes' address in Modbus frame, the limit should be around 256.But some say there is some limit on this scenario (RS485, Modbus-RTU) base on chips and wire. Chips support 32 ...
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320 views

libmodbus modbus_read_bits() returns illegal function

It is all okay when it comes to read registers with: modbus_read_registers(ctx,0,32,destination); It gives me values of 32 different registers. But when it comes ...
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3answers
436 views

RS485: Where to connect the cable shield in a battery-powered system?

I have implemented a Modbus with the RS-485 bus. The length of the bus is about 80 meters and I have connected the master and 10 slaves. The system is battery-powered and will be outdoor. My question ...
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1answer
141 views

My 3-node serial crossover bus works but I don't understand why

I have a three-node serial bus, wired into what I thought would be a master/slave configuration, but nodes that I didn't think would be able to receive data are receiving data. In other words, it's ...
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220 views

Modbus RS485 trouble

I'm trying to connect a power meter (http://www.ebay.co.uk/itm/272572704122) with Modbus over a RS485-USB converter (http://www.ebay.co.uk/itm/231570466384) The converter looks like a serial port (<...