Hello,

I am using a 1212 CPU, and programming in TIA V13 update 9.

My PLC is polling a MODBUS slave device.

On the MB_DATA_PTR input I set a pointer to my DB where the data must be stored.

This is working fine.

But, in MODBUS you can do only a poll of 100 registers, so I made a scanner that will change DATA_ADDR, but I need to be able to change also the location where the block will put the data.(the MB_DATA_PTR)

I want to say, with an integer, start writing in "MyDB" on register 200, then change that value to 300 and so on... (otherwise it's getting overwritten)

Does someone know's how to do it?

The structure of my DB are array's of 100 INT's each.

Thanks in advance.

Hi coldrestart,

In your suggested solution be sure that your update cycle will be too long even if the requested issue solved or not !! because in your way the same block request data from some registers then hold and call other registers then hold and so on and in every call it should store values read in different DBs, and that's not an efficiency.

So what about making different MB_Client blocks for each required register??

on the other hand, regarding your question .. you cannot change the pointer address dynamically as it must be in pointer form but you can make the following suggestion:

Compare the DATA_ADD manually as when you read from the register 100 then make a move operation for the data in the pointer DB to other new DB say DB1, and when you read from register 200 then make move to DB2 and so on.

// I didn't recommend this solution, but it was what you asked for.

Hoping to find this answer well,

Regards;

Ahmed Kadry

Hello Ahmed,

The goal is not to poll all the registers in one CPU cycle, it's a multiplexer that scans every block of registers at the time. Also, when you create for each series of registers a new block, you are filling you PLC with datablocks for no reason.So if I can change the destination adress, I don't need to make several instances of the MB_CLIENT.

The older modbus rtu block's had a DB_NO and DBB_NO input, those where offset's that you can set with an integer, it was every useful to point to a specific area in your DB.

Additionally, I think when you poll all block's together,with no multiplexing logic behind, I wonder if the modbus client can handle all those requests.

The DB block copy logic is indeed a (long) way.

Thank you for your advise!

Hello,

i had have the same application that i want to read and write more than 1000 register

i made an array  DATA_BUFFER [125]  of type WORD and i made a counter equals the number of my  read or write transaction.

and for each loop i change the parameters in the MB_Client instruction and  move the desired bytes from my data buffer to its destination (or write on it ) using MOVE_BLK intruction.

Thanks OpisOm!

Ok, I have chosen the multiple block call.

The advantage is that I can easy change the poll range,adress and read/write type.

Just some feedback for anyone how need to make this communication:

-When using multiple MB_CLIENT block's you need to trigger the DISCONNECT input, as otherwise the TCP connection remains open and other blocks won't work.

-Another important thing to know, by default Siemens will poll the 255 slave ID. Some modbus servers (slaves) have only Unit ID 1 (slave adress). You can change this by writing (using a simple move instruction) into the instance DB of the MB_CLIENT block -> MB_Unit_ID. There you can set you slave ID. Weird that Siemens didn't make it as a standard input on the MB_CLIENT block. (I know you device is already adressed with the IP adress and port, still some modules only respond on their proper slave adress)

Hello CR. I would really like to do just as you asked about in the beginning.

I would like to read register 1-10 and not 11-19, 20 to 39 and not 40 to 49, and so on, all the way up to 425.

Since one request only can be 125 words it would be very good to do multiple requests.

It would be really good to be able to point att different areas in MB_DATA_PTR in each loop.

Did yo find a solution for that or anyone else that have?

I've made a for loop in LAD:

No pointer, when there's a rising edge on the .DONE bit, I modify the start address.

Please criticize.