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please tell with one MBUS_MSG_P1 instruction how many register addresses can I access for reading values?

Created by: jeetendra at: 9/24/2010 2:54 PM (3 Replies)

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9/24/2010 2:54 PM	Rate (0)
jeetendra Experienced Member Joined: 8/30/2010 Last visit: 8/31/2024 Posts: 31 Rating: (1)	Thanks Robster,. One more problem I m facing that I am not got getting values at exact holding register addresses provided by siemens for MAG 8000. I am attaching one sample program for reading holding registers.Is it right or is there any another way? It works But please tell with one MBUS_MSG_P1 instruction how many register addresses can I access for reading values from MAG 8000. i.e.How much Max count value could I give in one instruction MBUS_MSG_P1 . Waiting for your reply. Thanks and Regards, Jeetendra Splitted from Reading 64bit value in S7200 PLC. Attachment Modbus _testing.zip (232 Downloads)
	Last edited by: O_Moderator at: 9/28/2010 10:07 AM new subject after splitting
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9/25/2010 9:45 AM	Rate (0)
jeetendra Experienced Member Joined: 8/30/2010 Last visit: 8/31/2024 Posts: 31 Rating: (1)	Hi, Please tell me How many registers addresses can access in one MBUS_MSG_P1 instruction. Because I have to access more than 60 parameters from MAG 8000 device. Regards, Jeetendra

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9/30/2010 12:22 PM	Rate (0)
danw Gold Expert Joined: 3/28/2010 Last visit: 11/30/2024 Posts: 1090 Rating: (215)	Your question on how many registers a software instruction can read is partially a function of the software, partially a function of the Mag8000 flow meter and partially a function of where the parameters are mapped. I can't help you any limitations on the software read function, but for the limitation on how many registers the Mag8000 can provide, see page 12 of that Modbus manual for details for the 03 function code: "The quantity indicates how many consecutive 16-bit registers are to be read. The quantity may range from 1 to 26 registers. If the quantity is greater than 26 an error code of 03 hex is returned in the exception response message." I interpret that as: an 03 query can expect up to a 26 register response, or up to 52 bytes (26 registers * 2 bytes/register = 56 bytes). The assumption (in using the 03 function code) is that all the requested registers are contiguous or sequential, one after another. The number of parameters you can read depends on the number of registers needed for those 60 parameters and whether those registers are within a block of contiguous registers. For 64 bit floating point values, each value consists of 8 bytes or 4 registers per floating point value. The Modbus manual provides the map (starting on page 21) which has a column for number (No.) of bytes for all the various parameters. It's just a matter of adding up the registers for your needed parameters. If the parameters you need are scattered across the map, it will obviously take multiple 03 function code reads to grab all the needed data.
	Last edited by: O_Moderator at: 9/30/2010 12:26 PM
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10/1/2010 8:28 AM	Rate (0)
jeetendra Experienced Member Joined: 8/30/2010 Last visit: 8/31/2024 Posts: 31 Rating: (1)	Very thanks, Whatever u have said is absolutely correct. I have observed that we could not read more than 26 registers in one response from device. Regards,

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