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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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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689 views

RxTx communication with multiple slaves using a RaspberryPi

I am a programmer trying to build a circuit with limited EE knowledge. With so many options(at least being new to this), just want to make sure I am going down the right path. What I am trying to ...
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1answer
359 views

RS485 not working in pic24fj128ga202

I would like to send and receive data between two PIC24F microcontrollers via RS485. Here i'm using SP3485 IC. I've set one microcontroller as transmitter and another one has receiver. My ...
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1answer
271 views

Is the information in MODBUS function code 16 redundant?

MODBUS function code 16 is "Write Holding Registers". The function command structure is: ...
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1k views

RS-485 Modbus device to MCU

I have a device (an energy meter) that sends data via RS-485 (Modbus protocol). And I want to collect it with MCU. The important thing is that I'm trying to design a board and I want it to be scalable....
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517 views

What is a conventional way to set a Modbus device address and baudrate?

What's a conventional way to set an address and baud-rate/parity on a Modbus device? Is this supposed to be done using hardware switches or can be done through a software? If done through a ...
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3answers
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Arduino as modbus master with MAX485 doesn't get any response

I'm having some troubles trying to query a modbus slave with an Arduino through RS485. I've already succeeded in querying a software modbus slave running on my PC through the USB/COM port using the ...
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2answers
330 views

FOSS ModBus libraries/VCl?

I'm looking for FOSS (for commercial use) VCL components for Delphi or libraries for Visual Basic/C/C++/C# ... basically, just something to abstract the h/w for development, although a h/w simulator ...
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1answer
901 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 ...
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Holding register in microcontroller for Modbus RTU

I'm very noob in microcntroller modbus. I want to do Modbus RTU slave device using RS232. I read modbus documentation from modbus.org site. But still I'm not clear about few things. How to define the ...
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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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94 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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9k views

Serial, RS232, Modbus, UART and TCP

I want clearly know this, Serial is protocol right? Then what is the Modbus? Is it under serial. And what are RS232 and RS485? What is UART and serial and UART same? What about TCP? TCP and serial ...
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2answers
3k 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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3answers
302 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
180 views

Get line to line values from line to neutral reading

My device can provide reading of Line to neutral values through Modbus , I'm requested to provide Line to Line readings which is not available in our Modbus table for the device. Is it possible to get ...
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3answers
2k views

Struggling with serial communication over RS 485 using modbus protocol

I have around 60-70 inverters that I want to talk to using arduino over RS-485. The connection that I am making with arduino is according to the attached image. Now I think that my connection part ...
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2answers
6k 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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1answer
206 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
45 views

Modbus Slave Firmware Deelopment

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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62 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
311 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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1answer
113 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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572 views

watchdog timer on PLC

I've currently set my PLC as a modbus slave, and written some python code which connect to it and act as a modbus client. I am having some problems with the modbus server sometime terminating the ...
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4answers
56 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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3answers
502 views

How to mount a DB-9 adapter to an enclosure

I have a DB-9 connector that needs to be mounted to an electronics enclosure. This hole for the DB-9 connector will be manually cut out. Question: How can the DB-9 connector be secured to the ...
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1answer
144 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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1answer
72 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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2answers
1k views

Difference between PuTTY Serial and Modbus

I am trying to communicate with a PM810 power meter using Modbus-RTU protocol with the RS-485 standard over 175 meters of CAT5e STP cable. It's less than ideal, but it's the situation I'm working with....
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2answers
856 views

Microcontroller Interface with Modbus Serial [closed]

I'm a beginner and I'm trying to acquire data from a microcontroller with Modbus serial protocol in order to create an interface with LabVIEW. I'm using a Prolific Technology USB-RS232 converter and ...
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2answers
347 views

Put firewall hardware with ethernet ports between serial ports

I want to put a firewall hardware product between a RTU and PLC to block some Modbus function codes (e.g. write coil, read coil). (This firewall product is able to do deep packet inspection on Modbus ...
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3answers
864 views

Arduino: RS232 Level Shifter or RS2323 Breakout needed? [closed]

I am trying to establish communications between an Arduino Mega2560 (Master) and a customised controller (Slave) via a Master-Slave RS232 link using Modbus RTU. The communication should be bi-...
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1answer
7k views

Reading MODBUS RTU slave device Registers via RS485

i have a panel multimeter that i use for my whole house that has RS485, Modbus RTU comunication. I know what registers to read in order to get currents, voltages, power factor etc How can i use this ...
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1answer
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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1answer
397 views

Modbus raw data COM capture file example?

I'm attempting to use a RS232 tap to sniff a serial connection between two devices that are communicating using Modbus RTU. I'm looking for any capture files taken from sniffing a COM connection or ...
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2answers
2k views

modbus_read_registers - ERROR connection timed out

We are using libmodbus library to read register values from energy meter EM6400 which supports Modbus over RTU. We are facing the following two issues. 1) We are facing issue with ...
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2answers
3k views

Connectors for a RS485 multipoint bus

I need to connect a bunch of Mass Flow Controllers on a RS485 (TIA-485-A) multipoint bus. I will use Modbus protocol to communicate with the devices. As specified on modbus.org (Specification and ...
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1answer
4k views

atmega32 communicating using rs-485 modbus protocol

I am trying to read values from a meter (WDM303FDWA1) through ATmega32. The communication to the meter has to be done by RS-485 modbus protocol. I would like to know what IC do I have to use to ...
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1answer
262 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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1answer
284 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
60 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
674 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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1answer
431 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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63 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
249 views

modbus extra byte at the end error

I'm working with MODBUS interface, based on a usb dongle which converts usb to RS485, then, in my board the rs485 half duplex is passed to max3362, as shown. The data is passed to NXP K64 controller. ...
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2answers
2k 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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1answer
51 views

Does FT232RLMG support RS-485?

I have the FT232RLMG device and a device that uses the RS-485 connection standard. I would like to know if I can connect the TX and RX pins of the FT232RLMG to the device that uses RS-485 directly or ...
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1answer
2k 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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1answer
698 views

Modbus RS485 - Voltage greater than 12V?

I have a boiler (Immergas Eolo Star) which has 2 pins where you can connect either a normal thermostat or a "remote control unit" which can also give more detailed information (self-diagnosis) about ...
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1answer
4k views

libmodbus trouble with reading from slave

I have the following code for connection to a temperature sensor as my slave using the libmodbus library in C, ...