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Modbus TCP Question about write request

Created by: Janson at: 5/8/2014 12:20 PM (8 Replies)

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5/8/2014 12:20 PM	Rate (0)
Janson Posts: 12 Rating: (0)	Hello, I have a quite simple question about how the write function is working. I have a IM 151-8 CPU which acts like client. I have no problem with reading from my third party, but I am not able to write to it. I have now scaled down my project so that I only want to write to one register. No reading at this point. The communication is established without errors. I do a new request when the last one is finished. Everything is working but I don't understand how I should write my command. To be able to give my third party an order I need to write the value A010 (40976) into register at place 5 in my DB. I belive it should look like following: L 11 T "A05_CONTROL_DAT".UNIT L 3 T "A05_CONTROL_DAT".DATA_TYPE L 5 T "A05_CONTROL_DAT".START_ADDRESS L 1 T "A05_CONTROL_DAT".LENGTH SET = "A05_CONTROL_DAT".WRITE_READ But where shall I put the value 40976? Please help a modbusbeginner! Best regards, Sara
Janson Posts: 12 Rating: (0)
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5/8/2014 12:48 PM	Rate (0)
Kaulquappe Posts: 3093 Rating: (324)	Hello Sara, the parameters are set correctly for a writing request. The Modbus FB takes the value out of the data DB with the responding offset. E.g. the registers in your data DB start with register 0, you write register 5, then the FB takes the value ofDBW8 and sends it. Regards, Kaulquappe
Kaulquappe Posts: 3093 Rating: (324)
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5/8/2014 1:54 PM	Rate (0)
Janson Posts: 12 Rating: (0)	Thank you for your answer! So, what can I do wrong? If I use Modscan and write the value 40976 at correct register, my third party respond correct. When I do it from the PLC nothing happens. No error or anything. As if it never got the message.
Janson Posts: 12 Rating: (0)
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5/8/2014 3:19 PM	Rate (0)
Wüstenräuber Gold Expert Joined: 10/3/2008 Last visit: 10/22/2025 Posts: 996 Rating: (129)	Hi, I assume the initialization is not correct. Which entries do you have in the parameter data block as start_x and end_x? Example 1: start_x = 0 and end_x = 10 => In DBW0 the register 0 is mapped, in DBW2 the register 1 is mapped and so on. You start an order for start_address = 5, length = 1 => The register 5 in DBW8 is transferred. Example 2: start_x = 1500 and end_x = 1510 => In DBW0 the register 1500 is mapped, in DBW2 the register 1501 is mapped and so on. You start an order for start_address = 1505, length = 1 => The register 1505 in DBW8 is transferred. If you want to transfer 40976, then it must be a part of the initialization. Example: start_x = 40976, end_x = 41000 Your order: start_address = 0, length = 1 => The register 40976 (which is mapped in DBW0) is transferred. The first register (= start_x) is always in DBW0. And from this the offset is calculated. Wüstenräuber
	Last edited by: Wuestenraeuber at: 5/8/2014 3:28 PM
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5/8/2014 4:03 PM	Rate (0)
Janson Posts: 12 Rating: (0)	According to the object acting as server it is adress 6 I want to change. (List starts from 1). If the server gets value 40976 at register 6 it will execute an order. The initial parameters are now set to: Start_1: 1 End_1: 10 Shall I change it the way you describe? Thank you!
Janson Posts: 12 Rating: (0)
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5/8/2014 5:40 PM	Rate (0)
Kaulquappe Posts: 3093 Rating: (324)	Hello Sara, I assume you are using the sample project. Open the variable table "client_job", trigger the job and monitor count_done and count_error. Which one increases? If count_error increases check the error number in save_STATUS. Regards, Kaulquappe
Kaulquappe Posts: 3093 Rating: (324)	Last edited by: Blue Moderator at: 5/8/2014 5:50 PM
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5/8/2014 11:49 PM	Rate (0)
Janson Posts: 12 Rating: (0)	It is count_done which is increasing. No errors..
Janson Posts: 12 Rating: (0)
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5/9/2014 10:21 AM	Rate (0)
Kaulquappe Posts: 3093 Rating: (324)	Well Sara, that's perfect. The value of DBW8 is sent to the communication partner. Regards, Kaulquappe
Kaulquappe Posts: 3093 Rating: (324)
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9/16/2015 1:47 PM	Rate (0)
CS Moderator Moderator Joined: 9/1/2005 Last visit: 10/31/2025 Posts: 4114 Rating: (195)	follow up question published by AutoGen_682437 is transferred to the discussion trying to see how modbus works with the ModScan and c300 Controllers. Best regards CS Moderator
	My Forum is the place to personalize your profile, track your topics, and maintain content that interests you. Best regards CS Moderator
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