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Read a local variable ( byte not word )

Created by: AmineOueslati at: 10/23/2012 7:25 PM (8 Replies)

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10/23/2012 7:25 PM	Rate (0)
AmineOueslati Posts: 31 Rating: (0)	Hi, I'm using S7-1200 as a slave and I need to read a local variable, a byte not a word, I'm using "CAS Modbus Scanner" simulator as a master How I have to configure the MB_SLAVE to be able to read a byte from it ??? please help
AmineOueslati Posts: 31 Rating: (0)
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10/23/2012 7:30 PM	Rate (0)
AmineOueslati Posts: 31 Rating: (0)	Here's a pic of the MB_SLAVE Attachment MB_SLAVE.jpg (124 Downloads)
AmineOueslati Posts: 31 Rating: (0)
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10/24/2012 11:10 AM	Rate (1)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 2/20/2025 Posts: 1554 Rating: (539)	You can't. Modbus deals in only 2 types of data: Booleans and 16-bit (one word) registers. The only possible ways are:- 1) Store a byte in the low byte of a 16-bit register (e.g. Byte value AAHex would be coded as 00AAHex in the register). Get the master to ask for the full register and then mask off the high byte by ANDing the register value with 00FFHex in the masters application layer. 2) Store the byte in the Boolean address space (coils / discrete input data type). Get the master to ask for 8 Booleans using function code 01 (read coils) or 02 (read discrete inputs), then get the application layer in the master to interpret this as a byte. Either way, it is not possible for the master to receive a byte without some extra work in the master's application layer.
	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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10/24/2012 12:27 PM	Rate (0)
AmineOueslati Posts: 31 Rating: (0)	Hello, Thank you Sir. If I want to read a boolean ( Local var). I've created a global DB in wich I've declared a struct (named data) that contains the boolean variables. Is it correct ? plz see attachement. What would be their address If I want to read them from the simulator ? regards,
AmineOueslati Posts: 31 Rating: (0)
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10/24/2012 3:32 PM	Rate (0)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 2/20/2025 Posts: 1554 Rating: (539)	It would seem from reading the S7-1200 manual that the Boolean data type access (coils / discrete inputs) can only be mapped to I and Q address space (not a DB or M space). So it ooks like you will have to use holding register address space (modbus addresses 40001+). You will read these using function code 3. It is very unlikely that you modbus simulator will have the ability to convert a word (register) to a byte. But I see no reason why the actual master device you will use when the project is shipped should not have the ability to read a register (word) and perform the required masking to convert it to a byte. In the meantime, it will OK to use the simulator for testing and just accept that you will see a word with the high byte value of zero and the byte value in the low byte (instead of an actual byte value per se). The testing will be just as valid.
	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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10/24/2012 3:54 PM	Rate (0)
AmineOueslati Posts: 31 Rating: (0)	Hello, Thank you for your help, I did as you said and here's what I get ( see attachement ) :/ regards, Attachment SIMULATOR2.jpg (102 Downloads)
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10/24/2012 3:55 PM	Rate (0)
AmineOueslati Posts: 31 Rating: (0)	The configuration of the request is in the attachement Attachment SIMIULATOR.jpg (100 Downloads)
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10/24/2012 4:40 PM	Rate (0)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 2/20/2025 Posts: 1554 Rating: (539)	What the slave has done there is to return what is known as an exception respose. It does this when the master tries to read a data address that is not present in the slave. I suggest that you read the modbus gernarel description document tip and get to know the modbus protocol a little better so you ca understand why it did this. The poll data 02 03 00 00 00 02 C4 38 decodes as:- Read 2 holding registers (4 bytes) from address 40001 (i.e. 40001 and 40002) from slave device 02. How big is your data DB? It will have to be a least 4 bytes long (2 words) otherwise, you will get this error returned. Remember you are asking for REGISTERS (2 bytes in size) not BYTES in the request.
	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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10/25/2012 9:07 AM	Rate (0)
O_Moderator Diamond Member Joined: 4/24/2009 Last visit: 8/20/2024 Posts: 2682 Rating: (135)	Thread closed, because question is already discussed in the following thread Local variable Transmission Best regards O_Moderator
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