Have problem reading single bit using modbus

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C#

can some1 tell me How to read single bit from com port using madbus protocol? in C#

Posted 5-Jan-13 19:41pm

Ravndra22

Updated 5-Jan-13 19:42pm

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Ravndra22 6-Jan-13 3:33am

In the referance you provided input reads in the form of bytes. I want to read single bit i.e high or low

5 solutions

Solution 1

Have a look at this article Simple Modbus Protocol in C# / .NET 2.0[^]

Posted 5-Jan-13 20:48pm

Jibesh

Solution 3

This will useful for you
http://msmvps.com/blogs/coad/archive/2005/03/23/SerialPort-_2800_RS_2D00_232-Serial-COM-Port_2900_-in-C_2300_-.NET.aspx

Posted 5-Jan-13 23:18pm

SajeeshCheviry

Solution 4

In Modbus RTU, you always read a byte. Naturally, in Modbus ASCII, you read characters, 8-bit characters, and each byte is a part of number represented as text:
http://en.wikipedia.org/wiki/Modbus#Frame_format[^].

So, you either read individual byte or some number and extract an individual bit from the data read. All software developers know how to extract a bit, but… do you need help?

—SA

Posted 6-Jan-13 7:04am

Sergey Alexandrovich Kryukov

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Ravndra22 7-Jan-13 0:42am

thnx all...

but problem is that I want to read 4 inputs from serial port in the form of ON/OFF. So what request messge I should write to serial port through modbus in order to read inputs?

like Function Code=2,slave address=1 and other?

Sergey Alexandrovich Kryukov 7-Jan-13 0:47am

You did not ask about request message. It depends on your device. The format should be known from the documentation. I answered your question, it should be enough. Are you going to accept it formally (green button)?
—SA

Solution 7

thnx CPallini. It really helped me.

Posted 8-Jan-13 0:29am

Ravndra22

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CPallini 8-Jan-13 6:34am

You are welcome.

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CPallini · Accepted Answer · 2013-01-06T22:22:00

Quote:
thnx all...

but problem is that I want to read 4 inputs from serial port in the form of ON/OFF. So what request messge I should write to serial port through modbus in order to read inputs?

You probably need reading 4 consecutive discrete inputs (or coils) from the device.
Have a look at MODBUS functions 0x02 (read discrete inputs) or 0x01 (read coils) explanations and samples in this document "MODBUS APPLICATION PROTOCOL SPECIFICATION"[^].

If you need the CRC computation routine, you may find the C source code at the end of this document: Modicon Modbus Protocol Reference Guide[^].

Have problem reading single bit using modbus

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