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Maximum data size per one Read/Write

Created by: Dolpoong at: 12/28/2015 2:05 PM (6 Replies)

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12/28/2015 2:05 PM	Rate (0)
Dolpoong Posts: 45 Rating: (0)	Hello, I will make Fault-tolerant based on S7-400H system. And the PLC should read from and write to a computer. Please refer attached diagram file. I am developing PLC side program, and our partner is in charge of programs running on the computer. Because MODBUS TCP is a popular communication protocol, our partner who is in charge of computer program would like to use MODBUS TCP for communication between PLC and the computer. Meanwhile, PLC and the computer should share comparatively large size data. The data size is above 2000 Bytes. PLC will perform MODBUS master. And I will assign one number(such as 20) of MODBUS address to the PC. I read a Siemens manual that says OpenModbus/PN library can be used. My question is as follows. 1. Can I send and receive 2000 Bytes of data per one read or write? 2. If I use CFC to use MB_REDCL, it looks like huge connecting work. Can I see proper example for this application? Many thanks and enjoy happy and wonderful holiday season. Attachment diagram-5.pdf (399 Downloads)
Dolpoong Posts: 45 Rating: (0)
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12/29/2015 9:49 AM	Rate (1)
Kaulquappe Posts: 3093 Rating: (323)	Hello Dolpoong, according to the Modbus specifiation the limits for one request are: - read 125 register - write 123 register - read 2000 bits - write 1968 bits Regards, Kaulquappe
Kaulquappe Posts: 3093 Rating: (323)
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12/29/2015 2:11 PM	Rate (0)
Dolpoong Posts: 45 Rating: (0)	Thank you for checking and listing answers to my question. When I read Siemens manuals such as "SIMATIC Modbus/TCP Redundant Communication via CP443-1 in H-Systems", I found out some example projects in CFC. I will use SIMATIC manager for my PLC logic design because target controller is S7-400H. My understanding is that CFC is a tool software for SCADA. In order to use MB_REDSV function block, should additional software be required besides SIMATIC manager to compose MODBUS client or server functions in PLC?
Dolpoong Posts: 45 Rating: (0)
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12/29/2015 3:20 PM	Rate (1)
Kaulquappe Posts: 3093 Rating: (323)	Hello Dolpoong, yes, you need the library 2XV9 450-1MB11, which is subject to license conditions. The demo version can be found here: https://support.industry.siemens.com/cs/ww/en/view/103474646 If you want to use the Ethernet interface of a PN CPU you need 6AV6676-6MB10-0AX0, the demo version can be found here: https://support.industry.siemens.com/cs/ww/en/view/103475102 Regards, Kaulquappe
Kaulquappe Posts: 3093 Rating: (323)	Last edited by: Kaulquappe at: 12/29/2015 3:21:19 PM Last edited by: Kaulquappe at: 12/29/2015 3:26:15 PM
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1/15/2016 5:28 AM	Rate (1)
danw Gold Member Joined: 3/28/2010 Last visit: 11/14/2024 Posts: 1074 Rating: (214)	1) > Can I send and receive 2000 Bytes of data per one read or write? No. One Modbus holding register = 16 bits = 2 bytes To write 2000 bytes from the PLC master to the computer slave means a total of nine separate read transactions (given assumptions below). One write transaction of 123 registers = 246 bytes per write transaction. 246 bytes/write transaction * 8 transactions = 1,968 bytes/write transaction, so a 9th write transaction would be needed for a full 2,000 bytes. That assumes a) that the PLC and the computer can handle 123 registers per write transaction. Many slaves and masters do not support the maximum numbers. b) that the PLC can pack two data bytes into a single Modbus register c) That the PLC can pack data into contiguous Modbus registers and the computer can unpack them on the other end. 2) > And I will assign one number (such as 20) of MODBUS address to the PC. A Modbus address = a Modbus register for integer data = 16 bits. The number 20 is not a very large number when you're talking about moving 2000 bytes of data.
	Last edited by: danw at: 1/23/2016 2:50:24 AM
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4/2/2016 12:43 PM	Rate (0)
Dolpoong Posts: 45 Rating: (0)	Dear, I posted a question regarding maximum transfer data size by OPEN MODBUS TCP MB_REDCL function. Now, I would like to implement 'MODBUS SERVER' function by using OPEN MODBUS TCP MB_REDSV function. I have not found length field parameter and maximum number restriction in case of MB_REDSV function contrary to MB_REDCL. Could I transfer 2000 Byte data per a MB_REDSV function call? Many thanks for your attention and comments.
Dolpoong Posts: 45 Rating: (0)
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4/4/2016 8:49 AM	Rate (0)
Kaulquappe Posts: 3093 Rating: (323)	Hello Dolpoong, the maximum data length is defined in the Modbus specification. So the is no difference if the S7 acts as client or server. Regards, Kaulquappe
Kaulquappe Posts: 3093 Rating: (323)
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