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MODBUS DATA converson in S7-1200 CPU

Created by: Vinay Pathak at: 9/2/2013 7:07 AM (4 Replies)

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9/2/2013 7:07 AM	Rate (0)
Vinay Pathak Advanced Member Joined: 6/7/2013 Last visit: 8/11/2024 Posts: 1 Rating: (0)	Hello Experts I am getting some data from modbus RTU in a form of hex coded double word in a db that data. In actual shows 5 point floating point we have an option of byte order in our our instrument in which we have selected 3-2-1-0 . Now according to this i am going through 32 BCD to dint and dint to real conversion for floting point . But we are not getting the actual figure of data which we are getting in local display of that equipmet. So can you please explain me how to convert this data in floting point. When we are taking this data in Allen Bradley PLC we are getting exect data because there is an option to convert data in float. Please provide me solution for S7 1200. We are using TIA v11.0 for programming that controller.

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9/2/2013 12:23 PM	Rate (1)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 12/5/2024 Posts: 1548 Rating: (538)	When interfacing with external systems, the real (floating point) data may be incompatible with the S7 system, due to endian issues (the byte / word ordering). This is a common issue when interfacing systems of differing manufacturers. Floating point numbers are encodeded in the IEEE754 format, which is 32-bits in size, and is therefore transmitted over the modbus interface as 2 16-bit registers. Now it is entirely possible that the data in these registers will require either the word order, byte order or possibly both swapping so that it is compatible with the S7 system. So say that the data comes into the S7 system as two register values AB CD. There are 4 possible permutations of swapping required:- AB CD (take data as supplied) CD AB (word swap only - this is the most likely) BA DC (byte swap only - not very common) DC BA (byte and word swap - not very common). So you will either need to get the remote device to swap the words over internally before it sends it over the modbus interface, or alternatively, read the data as 2 words and then swap it over in the S7 and then convert it to a real value. This thread contains details. Regards, Smiffy.
	Programming today is the race between software engineers building bigger and better idiot proof programs, and the universe producing bigger and better idiots. So far, the universe is winning...
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2/27/2017 6:32 PM	Rate (0)
positize Posts: 3 Rating: (5)	Here is a simple SCL routine I wrote to take two Modbus 16 bit words and generate a real (floating point) value. As far as I can determine, there is no way to do this in LAD (ladder logic): you must use SCL and use its DWORD_TO_REAL function (no LAD equivalent of DWORD_TO_REAL). You will need to determine the order of the two 16 bit words coming from the Modbus device: which word is the least significant word and which word is the most significant word. No way to know that order in advance: just have to experiment. Once you know that order, then call this SCL function with the two words read from the Modbus device to generate a real floating point 32 bit result from the two words. Declare an SCL function in your PLC named ModbusWordsToReal. The SCL FC will have two input values: LSWord - Data type 'Word' - The least significant 16 bits of the Modbus float value MSWord - Data type 'Word' - The most significant 16 bits of the Modbus float value The SCL FC will have one output value: RealResult - Date type 'Real'- The returned real value to use in PLC/HMI code The SCL FC will use one temporary value: Temp_1 - Data type 'DWord' - The two input words combined into one dword The SCL code is: #Temp_1.%W0 := #LSWord; #Temp_1.%W1 := #MSWord; #RealResult := DWORD_TO_REAL(#Temp_1); Attachment ModbusWordsToReal.jpg (1719 Downloads)
positize Posts: 3 Rating: (5)	Last edited by: positize at: 2/27/2017 6:35:41 PM Last edited by: positize at: 2/28/2017 5:11:51 PM Last edited by: positize at: 2/28/2017 5:15:25 PM Last edited by: positize at: 2/28/2017 5:17:45 PM Last edited by: positize at: 2/28/2017 5:19:19 PM
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This contribution was helpful to 4 thankful Users oswecCM Ahmet_Hazar FanaM fherlopezmx

7/23/2020 7:51 AM	Rate (0)
SimonFan Member Joined: 6/1/2020 Last visit: 3/16/2022 Posts: 1 Rating: (0)	Hi! all: Now i using the TIA Portal V15.1 already had the "CONV" instruction to transfer Dword or Dint to Real.

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5/31/2022 5:39 AM	Rate (0)
ManBz Member Joined: 4/9/2021 Last visit: 10/29/2024 Posts: 1 Rating: (0)	Thank you very much For me this method works very well. positize Here is a simple SCL routine I wrote to take two Modbus 16 bit words and generate a real (floating point) value. As far as I can determine, there is no way to do this in LAD (ladder logic): you must use SCL and use its DWORD_TO_REAL function (no LAD equivalent of DWORD_TO_REAL). You will need to determine the order of the two 16 bit words coming from the Modbus device: which word is the least significant word and which word is the most significant word. No way to know that order in advance: just have to experiment. Once you know that order, then call this SCL function with the two words read from the Modbus device to generate a real floating point 32 bit result from the two words. Declare an SCL function in your PLC named ModbusWordsToReal. The SCL FC will have two input values: LSWord - Data type 'Word' - The least significant 16 bits of the Modbus float value MSWord - Data type 'Word' - The most significant 16 bits of the Modbus float value The SCL FC will have one output value: RealResult - Date type 'Real'- The returned real value to use in PLC/HMI code The SCL FC will use one temporary value: Temp_1 - Data type 'DWord' - The two input words combined into one dword The SCL code is: #Temp_1.%W0 := #LSWord; #Temp_1.%W1 := #MSWord; #RealResult := DWORD_TO_REAL(#Temp_1);

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