Modbus RTU serial communication error

[Nectar_techabhi]

Hello Everyone,

I am facing an issue while communicating with a system using Modbus RTU protocol.

This system (master) sends 5 set of fixed queries one by one. No access to the master (pre-existing)software.

I have created Modbus slave response to this system as per the protocol in LabVIEW.

There are 3 such identical systems, out of which I was successful in communicating with one system but facing error in other two.

My device is connected to this system via Serial to RS485 converter.

I could read the queries sent by the master which is exact as defined

1) 01 03 00 00 00 51 84 36 H

2) 01 03 00 51 00 51 D5 E7 H

 and so on.

But the response I send to the queries is not reflecting on the Master.( The same which is working with one of the system).

The software in all the three systems are  identical as informed.

If there is any issue in the signal connectivity then I should have not received the queries.

Stuck!!

Can anyone suggest if I am missing anything?

Attaching Screen shot of my slave response

Thanks in Advance.

[PalanivelThiruvenkadam]

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@Nectar_techabhi wrote:

Hello Everyone,

I am facing an issue while communicating with a system using Modbus RTU protocol.

This system (master) sends 5 set of fixed queries one by one. No access to the master (pre-existing)software.

I have created Modbus slave response to this system as per the protocol in LabVIEW.

There are 3 such identical systems, out of which I was successful in communicating with one system but facing error in other two.

My device is connected to this system via Serial to RS485 converter.

I could read the queries sent by the master which is exact as defined

1) 01 03 00 00 00 51 84 36 H

2) 01 03 00 51 00 51 D5 E7 H

 and so on.

But the response I send to the queries is not reflecting on the Master.( The same which is working with one of the system).

The software in all the three systems are  identical as informed.

If there is any issue in the signal connectivity then I should have not received the queries.

Stuck!!

Can anyone suggest if I am missing anything?

Attaching Screen shot of my slave response

Thanks in Advance.

---

You have three different machines running similar Master Software and Your Slave Module running?

Have you tried available NI Examples to check whether it works fine, Also Compare the Installed Software in all three Machines.

[Nectar_techabhi]

---

@PalanivelThiruvenkadam wrote:

---

@Nectar_techabhi wrote:

Hello Everyone,

I am facing an issue while communicating with a system using Modbus RTU protocol.

This system (master) sends 5 set of fixed queries one by one. No access to the master (pre-existing)software.

I have created Modbus slave response to this system as per the protocol in LabVIEW.

There are 3 such identical systems, out of which I was successful in communicating with one system but facing error in other two.

My device is connected to this system via Serial to RS485 converter.

I could read the queries sent by the master which is exact as defined

1) 01 03 00 00 00 51 84 36 H

2) 01 03 00 51 00 51 D5 E7 H

 and so on.

But the response I send to the queries is not reflecting on the Master.( The same which is working with one of the system).

The software in all the three systems are  identical as informed.

If there is any issue in the signal connectivity then I should have not received the queries.

Stuck!!

Can anyone suggest if I am missing anything?

Attaching Screen shot of my slave response

Thanks in Advance.

---

You have three different machines running similar Master Software and Your Slave Module running?

Have you tried available NI Examples to check whether it works fine, Also Compare the Installed Software in all three Machines.

---

Thank you for your prompt response.

Yes three different machines have similar master software running as informed by the proprietor.

Unfortunately, cannot check much at the Master software side. Please suggest

The slave response I have attached is working fine on one of the three system.

I have checked  NI Examples and my slave response is working with MB Serial Example Master.vi.

I have also tried using termination resistor of 120ohm.

Please let me know if anything that I could verify further.

[PalanivelThiruvenkadam]

Any Hardware's Involved here? Do you have any Programming Manual available for Master Program?

[StevenD]

To clarify, you have three separate systems, each with one master and one slave. NOT three masters and one slave?

Also baud and the id can usually be changed pretty easily on the master. Can you at least verify that all the masters are 9600 and trying to read from ID 1?

[Nectar_techabhi]

---

@PalanivelThiruvenkadam wrote:

Any Hardware's Involved here? Do you have any Programming Manual available for Master Program?

---

The hardware involved is a serial interface card as shown below 

The remote terminal unit basically converts the RS485 signals and Ethernet and is connected to Master.

The serial interface card has jumper settings for half duplex and full duplex RS485 and also for termination resistor.

I had doubted the serial interface module, but confused because receiving query from master. If there was issue in hardware i think there should not be any query. Correct me if I am wrong?

I have received a communication protocol for interfacing with master. It has format of master query and for slave response.

I can not share the entire document but will sharing the communication settings and master query and slave response format.

Baud rate: 9600 bps
Parity: No parity
Data bits: 8
Stop Bit: 1
Offset in Holding Register Address : -1
Turn around time: 100 mSec

Master query for reading data of transmit groups

Command to Read data of	Slave Address	Function code	MODBUS Start Address Hi	MODBUS Start Address Lo	No of Registers to read Hi	No of Registers to read Lo	CRC Lo	CRC Hi
Transmit Group-G1	01H	03H	00H	00H	00H	51H	84H	36H
Transmit Group-G2	01H	03H	00H	51H	00H	51H	D5H	E7H
Transmit Group-G3	01H	03H	00H	A2H	00H	51H	25H	D4H
Transmit Group-G4	01H	03H	00H	F3H	00H	51H	74H	05H
Transmit Group-G5	01H	03H	01H	44H	00H	51H	C5H	DFH

Slave response format :

Response format for First tank group G1

Slave Address	1 Byte (Hex)	01H
Function Code	1 Byte(Hex)	03H
Byte Count	1 Byte(Hex)	A2H
Data-1 Hi	Higher Byte (Hex)	00H
Data-1 Lo	Lower Byte (Hex)	00H
Data-2 Hi	Higher Byte (Hex)	01H
Data-2 Lo	Lower Byte (Dec)	12
Data-3 Hi	Higher Byte (Dec)	01
Data-3 Lo	Lower Byte (Hex)	02H
Data-4 Hi	Higher Byte (Hex)	00H
Data-4 Lo	Lower Byte (Hex)	32H
Data-5 Hi	Higher Byte (Hex)	00H
Data-5 Lo	Lower Byte (Hex)	00H
Data-6 Hi	Higher Byte (Hex)	20H
Data-6 Lo	Lower Byte (Hex)	5DH
--	Data of 2nd UNIT	--
--	--	--
--	Data of 16th UNIT	--
Data-77 Hi	Higher Byte (Hex)	10H
Data-77 Lo	Lower Byte (Dec)	12
Data-78 Hi	Higher Byte (Dec)	16
Data-78 Lo	Lower Byte (Hex)	02H
Data-79 Hi	Higher Byte (Hex)	00H
Data-79 Lo	Lower Byte (Hex)	32H
Data-80 Hi	Higher Byte (Hex)	00H
Data-80 Lo	Lower Byte (Hex)	00H
Data-81 Hi	Higher Byte (Hex)	4CH
Data-81 Lo	Lower Byte (Hex)	59H
CRC Lo	Lower Byte (Hex)	As calculated
CRC Hi	Higher Byte (Hex)	As calculated

Looking forward to your reply.

[Nectar_techabhi]

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@StevenD wrote:

To clarify, you have three separate systems, each with one master and one slave. NOT three masters and one slave?

Yes I have three separate identical systems, each with one master and one slave.

Also baud and the id can usually be changed pretty easily on the master. Can you at least verify that all the masters are 9600 and trying to read from ID 1?

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Baud rate is 9600 and ID is 1 confirmed.

[StevenD · Accepted solution]

I think that's a pretty strong case for "not a software problem"

Check cables.