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MODBUS standard addressing

Created by: kijo at: 1/27/2017 10:32 AM (4 Replies)

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1/27/2017 10:32 AM	Rate (0)
kijo Posts: 79 Rating: (1)	Hi to all, I am doing program with MODBUS communication for the first time. According to manuals on this forum I manage to set everything I need to send/receive via MODBUS. (this program example helped me: https://support.industry.siemens.com/cs/document/47756141/how-do-you-establish-a-modbus-rtu-communication-with-step-7-(tia-portal)-for-the-simatic-s7-1200-?dti=0&lc=en-WW ) My PLC will be Slave and other company will do program for Master PLC. My problem is that I am not sure what is the best way to organize holding registers and how to made some list that will be easy to understand for programmer of Master PLC. Is it anyone here willing to share some example of MODBUS holding address list, I need to send 25 bits for alarm states, 10 bits for for indications, 4 real values (using 2 integers to send 1 real value), and receive 5 bits per pump for control, and one real value (temperature setpoint). I tested my program with another PLC as Master and everything is working, now I just want to be sure that I will make good address list so that others can pick up good data from right place from my PLC.
kijo Posts: 79 Rating: (1)
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1/27/2017 5:46 PM	Rate (0)
Navnag Platinum Expert Joined: 11/21/2007 Last visit: 3/16/2025 Posts: 2326 Rating: (309)	Hello kijo You can go through the Modbus Mapping Document of any standard Modbus slave device to get an idea of how to assign and prepare a easily understandable Modbus address Mapping. Attached pl find and Good ( Thorugh Detailed ) Modbus Mapping Example. Hope this helps Cheers Navnag Attachment ION_Devices_Modbus_Register_Map.pdf (1087 Downloads)
	There is no such thing as the last bug in a program There are no stubborn bugs, There are only stubborn Debuggers.
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This contribution was helpful to 3 thankful Users Sachinpatke kijo Duris_I

1/28/2017 7:51 PM	Rate (0)
danw Gold Expert Joined: 3/28/2010 Last visit: 3/19/2025 Posts: 1124 Rating: (219)	The idea of documenting is not to be minimalist and try to save space or words, the purpose is to let whoever has to use it understand what and where the data is. If you have tested your server/slave with another master/client, then you know where you put the data, so you know what the register addressing is. So open up a spreadsheet and make columns for - point tag/name (label) - register/address for the point - number of registers - format (boolean bit #, real/floating point, integer, whatever) - notes on things like range, units, multiplier/scaling factor, values that apply to the point And fill in the rows with the data. What the ION Modbus table example is missing is - a column for the indexed offset in hexadecimal, for example (4)0013 in zero based addressing is 000Ch, where (4)0001 is 0000h. for master/clients using zero based hex addressing - range for the value, like -100 to +100 or 0 to 2500. Not part of the Modbus messaging but it helps someone who needs properties for an OPC server. - the module label column is used for whatever module 1/2/3/4 are, and also engineering units. I'd call it 'eng units'.

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This contribution was helpful to 2 thankful Users kijo Navnag

1/30/2017 3:20 PM	Rate (0)
kijo Posts: 79 Rating: (1)	Dear danw, Nawnag, thank you for answers. In attachment you can find printscreen of my program, and MODBUS table with addresses. Something like this will be clear to understand for everyone? Did I miss something? Attachment MODBUS address list.pdf (817 Downloads)
kijo Posts: 79 Rating: (1)
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1/30/2017 4:48 PM	Rate (0)
Navnag Platinum Expert Joined: 11/21/2007 Last visit: 3/16/2025 Posts: 2326 Rating: (309)	kijo Did I miss something? I feel, you are on the right track. This is the right way to prepare a Modbus Mapping document. In addition, you also need to specify : Modbus Slave address , Baud rate , Parity, No of Data Bits, No of Stop Bits. To verify the Mapping, you should test your Configured slave using any standard Modbus Test Tool like Modscan / MBPoll etc. Cheers Navnag
	There is no such thing as the last bug in a program There are no stubborn bugs, There are only stubborn Debuggers.
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This contribution was helpful to 1 thankful Users kijo

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