Modbus RTU TIA PORTAL

Hello,

it is the 1st time that i need to establish a MODBUS communication between S71200 CPU1217 using CM1241 RS422/485 module and a NETWORK Analyzer instrument in order to read in my plc code values of voltage, current etc.

I have prepared PLC as Master and the Analyzer as slave. Analyzer has no choice for MAster - Slave in it's confuguration menu but we will see...

A critical problem is that it is not possible to make a real simulation in my office because the Analyzer is already installed at the plant.

The scope is : using PLC as Master i need to to read contunuously 85 Dwords from the Slave = Analyzer with adress 31.

There is no need to write to the slave anything, just to read.

Attached the 3 networks where i have prepared the plc code in TIA Portal.

1. Is it necessary to declare adress to Master ? or need to declare somewhere that plc is the Master ?

2. I need after the end of a data transfer, to start a new request. Is my plc code ready to do that ?

3. Is it able to read all the 85 Dwords with each request or must i do a trick ? According to TIA help only 122 words per query is allowable to read.

4. In DATA_PTR udt  i have declared the 85 Dwords that i have to read continously from the Slave - Analyzer.

5. In Modbus_Master module at input DATA_ADDR must be always the 1st memory position of Modbus 40001. Also in input DATA_LEN must put the value 170 ? And finally in input MB_ADDR must put the slave's adress ( 31 ) ?

Sorry for large explanation.

I hope this topic to help others too !

Hi.

1.
In your program used the MB_MASTER block, so the plc know it's the master. There are no hardware config anything like that for setting the plc as the master.

2:
Myself I would have done it a bit different, but I belive your code will work. You can look at the TX-light on the communication module. If that light flashes is a good indication that your program works correctly. 

3:
Yes, you need to do a trick. You need some kind of state machine that will split your query in 2. The first query will ask for the first 40 dwords, and the second query will ask for the remaining dwords. DATA_ADDR is then need to be set to 40001 + 40 = 40041.

4:
Sounds good.

5: 
Se answer to question 3. About the DATA_LEN, the first query the DATA_LEN need to be 80, and the second need to be 90. However you may split your queries different, just make sure that your queries is less then 124 words. MB_ADDR: Yes you are right, it should be 31 for your application. 

Hope I was able to help, and not to confuse you. :)

By the way, I see one bug in your software. In network 4, at the sekund "circuit":

If a error occur, then the query prosesss will halt. 

My suggestion:

Insted of:
"MODBUS_MASTER_DATA".MB_MASTER.DONE , 

You can use:
"NOT"  (inverted) , "MODBUS_MASTER_DATA".MB_MASTER.BUSY

Dear ola84,

thank you for very useful suggestions.

I will illustrate your suggestions and come back to check.

By the way, one general question about Modbus functionality :

To start reading from SLAVE, i must keeep the REQ input of MASTER module "1" until DONE or ERROR occurs ?..and then REQ must turn to "0" and agian to "1", to start a new REQ command cyclically in order to read continouolsy from the slave ?

Do i need to use time interrupt ?

In my application i must start the 1st query for 80 Dwords until receive DONE. Then send query for the rest 90 Dwords and when Master receives DONE, again request for the first 80 Dwords etc...cyclically without timer interrupt ?

No, REQ only need to be set true for one scan cycle, for starting a query. REQ can be set false while your program is waiting for the result from the slave device. When BUSY is set back false, then a new query can start. Use the DONE-flag to cheek that the result was revived correctly. 

Nope, no need of time interrupt.

Last question:
Yes, except a query can end with a error, so you need to check both DONE and ERROR. Or you can use the BUSY-flag.

Hello again dear ola84,

attached my code to read from slave 85 Dwords ( in 2 steps because of reading limit of 122 words )

Step 1 : reads the first 80 words ( 40 Dwords )

Step 2 : reads the rest 90 words ( 180 Dwords ).

I kindly ask your time, to check it if everything works ok and if possible to suggest me a more efficient way to handle this communication.

Do you see any erros in that code ?

Hi Kosmous.

I made some comments on the pdf, please take a look. The state machine is how I image it as well. 

If it's critical that you software works when you go to the plant, then I suggest you use another plc to simulate a modbus slave.

Your suggestion are right .

In your comments Reset of REQ is making when DONE is true may be ?

There are many ways to Rome ! :)

I am not sure if that will work or not, but I do know it will work if you set REQ false right away after the MB_MASTER block. I am worried that a new request will start before the new query parameters has been inserted. But I am not sure about this.

If you have a rs-232/rs-485 converter, you can connect a PC to the modbus line and "lisen" to the telegram with a modbus software.

One other thing you should be award of, is the endianness of your revived data.
https://en.wikipedia.org/wiki/Comparison_of_instruction_set_architectures#Endianness

Modbus is easy and light protocol, but there are a lot of pitfalls ! 

thank you for your time,

i will check it in real time during commisioning and come back to update the forum.