read/ write coils or discrete inputs through Modbus tcp

Hi there, I'm trying to understando what you said. So, if I want to read/write coils or discrete inputs through Modbus tcp, I still need a 40001+ address? 

It's not possible to read/write those coils/discrete inputs with the correspondants function codes?

Check my attachment.

The idea is to write a single coil. Everything seems good in the setup, no errors appear at all but it doesn't work. 

I've tested it with function code to write holding registers  and it works already. But that shouldn't be the proper way to do it.

Waiting for reply,

Rodrigo Domingues

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Split from Modbus/TCP discrete inputs.

Hello,

1. Hi there, I'm trying to understando what you said. So, if I want to read/write coils or discrete inputs through Modbus tcp, I still need a 40001+ address?

No. Why yes? Your address is Holding register and it has nothing to do with DI/DO.

2. It's not possible to read/write those coils/discrete inputs with the correspondants function codes?

Yes, it's possible.

3. Check my attachment. The idea is to write a single coil. Everything seems good in the setup, no errors appear at all but it doesn't work.

Your screenshot is with some errors and wrong parameters. From what device you're read/write data? Your Client block define MODE 105, in case of do tests with DI/DO you can use MODE 0 or 1 with LEN parameter. Next problem what I see is wrong init of DATA_PTR parameter. You have there alocation BYTE 1 but on right screenshot I see only 1 BOOL in structure.

4. I've tested it with function code to write holding registers  and it works already. But that shouldn't be the proper way to do it.

You mean handling with DI/DO via Holding register? If yes it can work but as you said, it's not proper way to do it. On other side I can't imagine HW device wjhat will use Holding register for DI/DO manipulation. As I recommended already somewhere else, it can be prepared between 2 PLCs f.ex. but it is not correct procedure due Modbus spec.

Duris_I

Hello,

1. Hi there, I'm trying to understando what you said. So, if I want to read/write coils or discrete inputs through Modbus tcp, I still need a 40001+ address?

No. Why yes? Your address is Holding register and it has nothing to do with DI/DO.

Because I've been trying to use the corresponding function code to its corresponding data type variable and it's not working. The only way I can see some changes is by using holding register function code and I can change bool but it's the same like changing a variable type word bit by bit. (Which, like we agree, it's not the proper way to do it).

3. Check my attachment. The idea is to write a single coil. Everything seems good in the setup, no errors appear at all but it doesn't work.

Your screenshot is with some errors and wrong parameters. From what device you're read/write data? Your Client block define MODE 105, in case of do tests with DI/DO you can use MODE 0 or 1 with LEN parameter. Next problem what I see is wrong init of DATA_PTR parameter. You have there alocation BYTE 1 but on right screenshot I see only 1 BOOL in structure.

I have 2 cpu 1500's series which one I defined it as Client and the other one as Server and applied Client block  for the Client CPU and Server block for the Server CPU.

The configuration Setup I've done it the same by most  (if not all of them) of the websites and videos out there. And for Holding registers command, I'm able to send and receive data. My problem is the boolean variables and to change their state with modbus communication. 
I have my variables inserted in a Global DB without optimized access. This DB has the same number and type of variables in both cpu's. 

Using MODE 105 or MODE 1 is the same (or should be the same) if I use the MB_Address and Length correctly. But none of them seems to be working. 

DATA_PTR is the way it is because in the MB_Server block the MB_HOLD_REG if I put P#DBx.DBX0.0 BOOL 1 and the structure with a single boolean in it, when I go online it gives me the 16#8187 error and it means: "The MB_HOLD_REG parameter has an invalid pointer. Data area is too small.".  But even if I put an array of 8 Boolean variables and I put pointer like: P#DBx.DBX0.0 BYTE 1. It still say the same error.

I can't use DB's if I want to read/write coils? I've been trying everything to work, not sure what I'm missing.... If you could send some working example with read or write coils, so I can try to see what I missed, I would appreciate it.

Hello,

I can recommend to you only one - pls learn more about Modbus and definition (data types available) of each registers types. You can then easy find where you do mistakes and I can tell you that you do mistakes on both sides. Good to see after while that you have 2 PLCs network, then this problematic is more clear now. I can give you one tip, it's simply easy due specific register type what data type it can use, then you can correct also your Server code. You ask, I have defined data type bool, byte and it doesn't working. My tip again, what data type offer Holding register.

Duris_I

Hello,

I can recommend to you only one - pls learn more about Modbus and definition (data types available) of each registers types. You can then easy find where you do mistakes and I can tell you that you do mistakes on both sides. Good to see after while that you have 2 PLCs network, then this problematic is more clear now. I can give you one tip, it's simply easy due specific register type what data type it can use, then you can correct also your Server code. You ask, I have defined data type bool, byte and it doesn't working. My tip again, what data type offer Holding register.

 Modbus communication protocol  knowloedge isn't the problem. I know how it works and what type of data is allowed to each function code available. 

The parameters setup with the goal to read or write holding registers works. 

 i.e.   (Writing 2 Holding Registers, starting with address 1, and both db's in Client and server have 5 variables type Word)---------- 2 bytes * 5 = 10 bytes of lenght)

MB_MODE - 116                                                 or 1
MB_ADDR - 1                                                     or 40001 
MB_LENGTH - 2
MB_PTR - P#DBx.DBX0.0 BYTE 10

And it did work


I just tried to do the same for read or write coils by applying the same logic.

i.e.   (Writing 2 Coils, starting with address 1, and both db's in Client and server have 8 variables type Bool)---------- 1 bool * 8 = 1 byte of lenght)

MB_MODE - 115                                                  or 1 
MB_ADDR - 1                                                      or 00001
MB_LENGTH - 2
MB_PTR - P#DBx.DBX0.0 BYTE 1



But it doesn't work already. I just can't see what am I doing wrong comparing the 2 types of requests where the only thing to do is change the function code  to the respective 1, as well as the address. 

If you could help me by pointing out directly to what you believe seems to be the problem I would appreciate it.

Best regards, 

Rodrigo Domingues

Hello,

very sry:

user

Modbus communication protocol  knowloedge isn't the problem. I know how it works and what type of data is allowed to each function code available.

 If you know how it works then you must also know that Holding register defines as minimum WORD data type register, then maybe signed/unsigned and also after that 32 bit registers types. Take it as base system, then after your info that Modbus is known to you I really don't understand to your single bool or single byte Holding register data type definition on Server side. Hope now you understand this and can change your server block to proper DB with expected data type. You simply also can't read/write your Server from Client with your single bit/byte Holding register. More clear now?

Look again pls to your eaxmple that is working -> Server has defined 5 WORD HR (hope I understand it ok from your post), here is simply clear that it must work because due your expected LEN=2 (you want read 2 registers), your server buffer is enough long ;)

Hello,

next one:

user

just tried to do the same for read or write coils by applying the same logic.

i.e.   (Writing 2 Coils, starting with address 1, and both db's in Client and server have 8 variables type Bool)---------- 1 bool * 8 = 1 byte of lenght)

MB_MODE - 115                                                  or 1
MB_ADDR - 1                                                      or 00001
MB_LENGTH - 2
MB_PTR - P#DBx.DBX0.0 BYTE 1

 here is clear that you (very sry) don't understand to Modbus functionality because Coils are physical DO on PLC in this case, not any DB as you expect probably.

Duris_I

Hello,

very sry:

user

Modbus communication protocol  knowloedge isn't the problem. I know how it works and what type of data is allowed to each function code available.

First off, I would like to start by apologizing and to thank you because I didn't see that the Holding Register parameter has a minimum defined data type regist. Which make sense to majority of my mistakes. But also I saw this written in the description of MB_HOLD_REG: "The holding register contains the values that may be accessed by a Modbus client using the Modbus functions 3 (read), 6 (write), 16 (multiple write) and 23 (reading and writing in one job)."

So, if I'm not mistaken, the only way to read/write coils is through holding register function codes (03, 06, 16). and then I'm basically setting a Word/2 byte variable just I'm doing it bit by bit. 

So how can I read or write some coils status using the functions code (01, 05, 15) if the only way to access to DB pointed in MB_HOLD_REG is with function code 3,6,16?


I hope I could make my point,

Best Regards,
Rodrigo Domingues