Hi All,

I am a beginner to Modbus communication so struggling a bit with this one.

I have seen the sample code and HWConfig but I cannot understand how I would Read & Write to the one address. I have configured my HW to use DB22 and input this into the parameters for FC3,6,16 but I don't understand how I configure which Modbus address it is referring to. 

I need to be able to read and write to Modbus address 41538 on a device, do I have to configure a large array of data mapping from 40001 all the way to 41538 or is there a way to just read/write to 41538?

I am using a CP 341-RS422/485 card on a CPU 314C-2 PN/DP processor.

Any help is gratefully received.

Regards,

Ian

Duris_I

Hello,

no you don't need create big array in case you want read only one register. In this case you need only pay attention if you need read f.ex. REAL value from device because it takes 2 WORDs, so in this case you need create 2 WORD data type structure or similar array. Number of elements in recv DB depends to how much registers you want read with one request (LEN parameter you need set correctly) so to this you need think about max elements in your DB.

 This is for a Modbus Master and mine is a slave, I don't know how to specify the Modbus address that I am reading/writing to. I know how to configure the parameters and have entered the DB's I am using but how do I configure the Modbus address, is this done during the driver download or in some configuration somewhere?

Ian

Duris_I

Hello,

better to explain for first clearly what system your CP341 use. Look to attachment of Slave system, page 42 explain how to use more options, also with cooperation page 24 how define access.

 Sorry to keep bothering you but I am showing how stupid I can be :-)

If I have Modbus address 41538 how does this reflect in DB number? I am still struggling to understand the correlation between PLC DB and Modbus Address?

Regards,

Ian

Hello Ian;

Let's analyse the relationship between Modbus addresses and the Modbus slave S7 address with the Modbus slave driver for CP 341.

First point, you want to access Modbus address 41538, this would imply your master is using Modbus function 03 (read output registers).

You  must then set a DB number as "Base DB number" for FC 03,06, and 16 See the manual: you must use the Parameter assignment tool to do so (as suggested in the manual). Th e tool is a separate software that you receive with the Modbus software for CP 341, or that you can download here on the Product Support (use the Search box at the top of this page)

Once that is done, and say you have selected DB100 as the Base DB number, DB 100.DBW0 to DB100.DBW1022 will correspond to the first 511 Modbus output registers starting with address 40001 (see the screenshot below for an example).

So, let's start the analysis (with Base DB number set at 100):

Modbus address    ------------------------------->  S7 address

40001                                                                DB100.DBW0

40511                                                                DB100.DBW1022

.....

40512                                                                DB101.DBW0  

41023                                                                DB101.DBW1022

......

41024                                                                DB102.DBW0 

41535                                                                DB102.DBW1022    

.....

41536                                                                 DB103.DBW0

41537                                                                 DB103,DBW2

41538                                                                 DB103.DBW4

......

42047                                                                 DB103.DBW1022

Note that on page 82 you will find an equation that will make this analysis moot: Practice with it until you can easily convert from a Modbus address to a DBxxx.DBWyyyy address in the S7 memory.

Hope this helps,

Daniel Chartier