RS485 Request and Answer

[imranahmed]

Please let me learn about sending RS485 Modbus RTU request via Arduino UNO and get answer from device.
I have written method for sending request and getting answer but I want to know somethings,
there is a lookup table how to use it and make a request in Arduino?
Polynomial is 0xA001.
Please see attachments.

 

[imranahmed]

How to calculate float (4x bytes little endian: a[6], a+1[5], a+2[4], a+3[3] = 229.875519)?

 

[FvM]

MODBUS has no standarized float format, it's manufacturer specific. Presume in case of doubt regular IEEE format, identical to float format used by most C compilers.

To implement MODBUS on Arduino, you can use existing library.

 

[imranahmed]

MODBUS has no standarized float format, it's manufacturer specific. Presume in case of doubt regular IEEE format, identical to float format used by most C compilers.

To implement MODBUS on Arduino, you can use existing library.

Please recommend me any library I used ModbusMaster but I have some confusion for retrieving data from device.
I have to get analyser readings from its RS485 port.

 

[FvM]

I'm not yet using MODBUS on Arduino and can't suggest a specific library. I however implemented MODBUS master and slave on different platforms. I suggest to review the various available Arduino MODBUS libraries.

As for your general questions regarding "retrieving data", they are answered in the MODBUS APPLICATION PROTOCOL SPECIFICATION downloadable at modbus.org. I'm not sure what your application is. Is it a regular MODBUS Client ("Master")?

--- Updated Jan 21, 2022 ---

CRC implementation is described in MODBUS over serial line specification and implementation guide

The most recent specs are

https://modbus.org/docs/Modbus_Application_Protocol_V1_1b3.pdf

https://modbus.org/docs/Modbus_over_serial_line_V1_02.pdf

 

[imranahmed]

I'm not yet using MODBUS on Arduino and can't suggest a specific library. I however implemented MODBUS master and slave on different platforms. I suggest to review the various available Arduino MODBUS libraries.

As for your general questions regarding "retrieving data", they are answered in the MODBUS APPLICATION PROTOCOL SPECIFICATION downloadable at modbus.org. I'm not sure what your application is. Is it a regular MODBUS Client ("Master")?

--- Updated Jan 21, 2022 ---

CRC implementation is described in MODBUS over serial line specification and implementation guide

The most recent specs are

https://modbus.org/docs/Modbus_Application_Protocol_V1_1b3.pdf

https://modbus.org/docs/Modbus_over_serial_line_V1_02.pdf

I have a Energy Analyser which displays Volt, Current and lot of other readings, it has RS485 port on back side showing AB, I want to retrieve its reading
via Arduino to display on Serial Monitor.

Attached is Energy Analyser`s Modbus Registers ducument, please see last of document.

--- Updated Jan 22, 2022 ---

How to calculate?

 

[KlausST]

Hi,

You ask how to calculate the decimal value of a 32 bit float.

My answer:
I'd not calculate it manually.
The straight forward solution is to define a struct for the Modbus answer.
(Good thing here is that the answer almost always is a float).
Struct:
* uint8 a_ID
* uint8 a_func
* uint8 a_count
* float a_value
* uint16 a_crc

Then just use a_value as you like (calculations, compare, form into a string...)

Klaus

 

[FvM]

How to calculate?

Why didn't you try to do the calculation yourself according to IEEE float format specification during the last three days?

There are also online calculators, e.g. this https://www.h-schmidt.net/FloatConverter/IEEE754.html
The first hit when seaching for "ieee float calculator".

Typing in the example number clarifies that your device uses standard 32 bit float format

Respectively you get the value by casting 4 bytes to float. Just standard C programming.