MB_Master Help

Hi,

Im learning about Modbus technology blocks. And i have small problem.

• Can somone explain how does MB_MASTER create frame ? I want to send something like in the picutre and I fully dont understand the inputs of the block for me its :

REQ = Request

MB_ADDR = ID of the slave device

MODE = 0 if we read and 1 if we write

DATA_ADDR = Starting adress, so if the manual of the device says that Inverter Frequency is on 2001H i put 16#2001 

DATA_LEN = Its the length of how much i want to take from my buffor DB to the memory of the device DATA_PTR = My buffor where i prepare place for saved data or data for send

Hello,

yes, 03 is Slave address, MODE is 1 (FC6 is write one HR), HR address is dec 48193 not 16#2001, you need build this as TIA help shows, simple math 40001+8192->48193, LEN is 1 and value for write is 1. You can easy check this as send request from PLC and monitor on PC MB RTU application as Slave, incomming message. Look to attachment.

So if i want reach more memory then :

• Use same starting adress point in this ex. 48193
• Change DATA_LEN for the amount that i want ex. 2 insted of 1
• And configure my DATA_PTR DB to store values in 2 words ( do we always say taht 1 register = 1 word ? so then the devices are limited to use 65534 word on one MB_MASTER) 

Also instruction says that bit operation use values from 1 to 9999 so previous example in theory i can do in two ways 

1. Your represtantation
2. Starting point us ex. 10 - > DATA_LEN 1 -> DATA_PTR simple bool value.  (Then if i want to read or write more values i just change DATA_LEN and my instace od DATA_PTR )

Am i right sir? 

P.S Sorry for kinda dumb questions i feel like this is old fashioned protocol and its hard to find good source of information 

Hello,

I recommend again use TIA your version manual with expected block used, MASTER/SLAVE/CLIENT and check that table where is described what parameters use for specific regsiter type. As next helpful web site I recommend www.simplymodbus.ca where you can easy improve knowledge. Now to your questions.

1. Use same starting adress point in this ex. 48193

Yes, in case you want read from this address and also this register value.

2. Change DATA_LEN for the amount that i want ex. 2 insted of 1

Yes, this value is ok in case your buffer is long enough with expected data type and expected length read/write..

3. And configure my DATA_PTR DB to store values in 2 words ( do we always say taht 1 register = 1 word ? so then the devices are limited to use 65534 word on one MB_MASTER)

Yes, 1 register 1 WORD in case you manipulate with Input and Holding registers. I recommend read TIA help/manual where is clear how much registers due type can MASTER block read at once. It is not 65534.

-------------------------------------------------------------------------------------------------

1. Also instruction says that bit operation use values from 1 to 9999

in this case Coils not Inputs.

2.Your represtantation

no. My representation shows Holding register Write function, no bit operation function code. MODE can be same value depended Read/Write but destination address is different, no offset 40001 but 1.

3. Starting point us ex. 10 - > DATA_LEN 1 -> DATA_PTR simple bool value.  (Then if i want to read or write more values i just change DATA_LEN and my instace od DATA_PTR )

This is 9-th Coil status (Output on PLC f.ex.) but you need be careful here to correctly interpret this because Output addresses on PLC are addressed with 8 long (0.0-0.7 then 1.0-1.3 f.ex.), logically. You can try read bool data with existing WORD PTR defined buffer and you can easy see that is possible, you need have everytime long enough buffer for your expected LEN of type and buffer data type defined. It's easy to understand and count.

Hello,

some time passed, any news? Are my informations helpful to you?

Hi,

Had an winter holiday trip this week im going back to testing etc :D

Hi,

Your guide was very helpful i tested some data exchange on inverter and it worked without anyproblems

Thanks