8/21/2020 4:52 PM | |
Posts: 25 Rating: (0) |
How can I get a reading (volume flow for example) from a transmitter connected over modbus rtu? Thank you for your help. |
8/21/2020 6:21 PM | |
Posts: 25 Rating: (0) |
Thank you for your reply. The instrument model is 7ME3570-1JA40-0AA1 PLC is 6ES7417-4HL04-0AB0 The plc is connected to ET200S with profibus over 6GK7443-5Dx04-0XE0 and the transmitter is connected to ET200S with modbus over 6ES7 138-4DF11-0AB0 input card. https://support.industry.siemens.com/cs/ww/en/view/109769365 |
Last edited by: neman at: 08/21/2020 19:34:46Last edited by: Jen_Moderator at: 08/24/2020 08:22:03Optimized link. |
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8/22/2020 4:07 PM | |
Joined: 9/27/2006 Last visit: 9/22/2024 Posts: 12282 Rating: (2685) |
Hello neman; You will have to look at a few documents before you are ready to establish the link between your flowmeter and your 1SI module over RS 485. Connections over RS 485 are described in the screenshot below, you will have to make sure all other serial parameters match between the master and the s;ave. First: your 1SI module will be operated over a Profibus-DP link connected to a CP 44-5 Ext. This requires you use special functions to be installed in the S7-400 program: Serial Communication with ET 200S 1SI Module via PROFIBUS/PROFINET CP https://support.industry.siemens.com/cs/ww/en/view/26263724 Chapter 3 of the 1SI module manual addresses Modbius communication, you will have to study section 3.5 for information on FB 3 (S_Send) and FB 2 (S_RCV) for Modbus master transmissions, and make sure you understand the mechanism of the SEND_DB for your Modbus master requests (this is where you address a single slave and request one or more registers to read): 1SI Modbus RTU master Now, in the manual you sent me earlier for the FST020, you will see in appendix A a descriptiuon of Modbus communication parameters and registers for this transmitter. Section A9.2 describes process registers (as ^part of Holding registers that start with address 40000); Volume flow is register 3000, and is a Floating-point value (occupying 4 bytes) So, if you have a basis in Modbus exchanges you will understand you must use Function 4 (to read Holding registers) with an offset of 3000, and request 2 words (length) to receive this value. In your S7 program you will have to send this to a REAL variable (floating-point). If the value received is strange, you might have to change the byte order of the register, how this is done can be seen in A6. Hope this helps, Daniel Chartier |
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