Register reading with Modbus TCP

[zzmMG]

Hello all,

I was able to successfully connect rapidscada 6.4 to a modbus TCP device (power analyzer).
Now, I want to read data from registers lika voltage, current …
How am I supposed to adjust values in the appropriate channel?
The device documentation says following as example:
Request:
Address: 0A, Peripheral number: 10 in decimals.
Function: 04, Read function.
Initial Register: 0000, on which the reading will start.
No. of Registers: 0002, number of registers read.
CRC: 70B0, CRC Character.

expected response:
Address: 0A, Responding peripheral number: 10 in decimals.
Function: 04, Read function.
No. of bytes: 04, No. of bytes received.
Registers: 0000084D, value of the phase voltage of L1: VL1 x 10 : 212.5V
CRC: 8621, CRC Character.

Any documentation will be appreciated.
Thanks for your help

[manjey73]

In input script – Cnl/10

[manjey73]

In configuration Modbus – group – Input Registers.
Value – 4 bytes uint.
Maybe change byte

[Mikhail]

Hello,
This video may help.
If you add links to the screenshots of your settings and data, it would be useful to resolve the questions.

[zzmMG]

Thanks. Now I am progessing.

I cannot get min / max value changed (or read?), min in registers 164-165 and max 106-107

I use configuration Modbus – group – holding Registers – Value – 4 bytes uint for both element and command

following result of command:

Send: 00 0e 00 00 00 0b 01 10 01 64 00 02 04 00 00 08 98
Receive: 02 07 00 00 00 06 01
Receive: 10 01 64 00 02
OK

[Mikhail]

Try the Modbus parser https://rapidscada.net/modbus/ to find mistakes in requests.

[zzmMG]

My bad, the register i was trying to write to was not meant to be written but reset only. The parser helped a lot.

Thx

[rapidscadaaks]

Hi
I have a Waveshare Modbus RTU(D) Relay with has 8 channels each of Digital Inputs and Outputs.
I am able to use to digital outputs using commands and also check their status.

I am however not able to read status of Digital Inputs. Waveshare manual gives following parameters for reading input status:
Modbus Function Code 0x02
Address (HEX) 1×0000 …… 1×0007 for the 8 channels

I have tried various combinations and have not succeeded

I will be grateful for assistance

[rapidscadaaks]

Details & manual of the device:

https://www.waveshare.com/wiki/Modbus_RTU_Relay_(D)

[Mikhail]

Hi,
Please copy a part of the communication line log to check data packets, and provide screenshots of the Modbus template.

[rapidscadaaks]

Hi
I have the screenshots ready. Please advise how to attach as I do not seem to find option for uploading file

[rapidscadaaks]

https://drive.google.com/drive/folders/1vlIcl4xIIcAInInh3mefr4DbJkR56pO0?usp=sharing

I hope the link is fine

Thanks

[Mikhail]

Hi,
Could you copy the packets from the communication line log as text?
They can be found in a text file in the logs directory.

[rapidscadaaks]

Hi
In 100% cases, Digital Input is failing

2025-07-23 18:10:48 Session with the device [47] Waveshare_Relay_Test
Request element group “Relays”
Send (8): 01 01 00 00 00 08 3D CC
Receive (2/2): 01 01
Receive (4/4): 01 00 51 88
OK
Request element group “Digital Input”
Send (8): 01 02 00 00 00 08 79 CC
Receive (0/2):
Error: communication failed
Request element group “Digital Input”
Send (8): 01 02 00 00 00 08 79 CC
Receive (0/2):
Error: communication failed
Request element group “Digital Input”
Send (8): 01 02 00 00 00 08 79 CC
Receive (0/2):
Error: communication failed

2025-07-23 18:10:52 Session with the device [47] Waveshare_Relay_Test
Request element group “Relays”
Send (8): 01 01 00 00 00 08 3D CC
Receive (2/2): 01 01
Receive (4/4): 01 00 51 88
OK
Request element group “Digital Input”
Send (8): 01 02 00 00 00 08 79 CC
Receive (0/2):
Error: communication failed
Request element group “Digital Input”
Send (8): 01 02 00 00 00 08 79 CC
Receive (0/2):
Error: communication failed
Request element group “Digital Input”
Send (8): 01 02 00 00 00 08 79 CC
Receive (0/2):
Error: communication failed

2025-07-23 18:10:56 Session with the device [47] Waveshare_Relay_Test
Request element group “Relays”
Send (8): 01 01 00 00 00 08 3D CC
Receive (2/2): 01 01
Receive (4/4): 01 00 51 88
OK
Request element group “Digital Input”
Send (8): 01 02 00 00 00 08 79 CC
Receive (0/2):
Error: communication failed
Request element group “Digital Input”
Send (8): 01 02 00 00 00 08 79 CC
Receive (0/2):
Error: communication failed
Request element group “Digital Input”
Send (8): 01 02 00 00 00 08 79 CC
Receive (0/2):
Error: communication failed

2025-07-23 18:11:00 Session with the device [47] Waveshare_Relay_Test
Request element group “Relays”
Send (8): 01 01 00 00 00 08 3D CC
Receive (2/2): 01 01
Receive (4/4): 01 00 51 88
OK
Request element group “Digital Input”
Send (8): 01 02 00 00 00 08 79 CC
Receive (0/2):
Error: communication failed
Request element group “Digital Input”
Send (8): 01 02 00 00 00 08 79 CC
Receive (0/2):
Error: communication failed
Request element group “Digital Input”
Send (8): 01 02 00 00 00 08 79 CC
Receive (0/2):
Error: communication failed

2025-07-23 18:11:04 Session with the device [47] Waveshare_Relay_Test
Request element group “Relays”
Send (8): 01 01 00 00 00 08 3D CC
Receive (2/2): 01 01
Receive (4/4): 01 00 51 88
OK

[Mikhail]

Hi,

According to Modbus Parser, the following data is requested:
https://ibb.co/bjB7xMyC

Does it match the Modbus register map?

[Author]

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