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Modbus connection on 1217c

1/5/2018 9:57 AM	Rate (0)
Milancho Member Joined: 4/14/2016 Last visit: 11/23/2021 Posts: 4 Rating: (0)	Hello, I'm using S7-1217C (6ES7 217-1AG40-0XB0), V4.2 with TIA Professional V14 SP1. Is it possible and how to make a ModBus RTU connection using the integrated differential I/O on the 1217, namely DI b.2 - b.5 and DO a.0 - a.3. Thank you in advance Best

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1/5/2018 10:33 AM	Rate (0)
Duris_I Diamond Expert Joined: 4/28/2015 Last visit: 2/24/2025 Posts: 10832 Rating: (836)	Hello, for this situation you have available CB1241 or CM1241 modules. Look to your CPU type specification again, mainly about Voltage levels. Update: just one example (link): S7-1217 Differencial output levels Example
	Last edited by: Duris_I at: 1/5/2018 10:47:51 AM With best regards... Ivan Duris Flight: OMICO
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4/29/2022 2:19 PM	Rate (0)
TomasGarcia Member Joined: 2/1/2019 Last visit: 3/19/2024 Posts: 3 Rating: (0)	Have you found an answer? Greetings

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5/1/2022 2:52 PM	Rate (1)
danw Gold Expert Joined: 3/28/2010 Last visit: 3/13/2025 Posts: 1123 Rating: (219)	In a world with economic consequences, I am sure that the answer the original poster (OP) found was "No", simulating Modbus RTU with discrete I/O is not practical in any sense. On another forum, I once joked that the format of the Modbus protocol, half duplex master/slave and the Modbus function code syntax, could be accomplished on the hardware bus side with smoke signals. But no one does smoke signals, because it's impractical and has economic consequences. Simulating Modbus RTU with discrete I/O is as impractical as using smoke signals. PLCs are used in commercial activities that require reasonable costs to engineer, purchase, commission and support. Using discrete I/O to simulate either point-to-point communication over RS-232 or multidrop communication over RS-485 meets none of the commercial requirements, none. For one, Modbus RTU is distinguished from Modbus ASCII by its fairly tight timing requirements. I seriously doubt that a PLC CPU can execute the discrete I/O timing needed for Modbus RTU communications that is so easily accomplished with the dedicated design and implementation that a comm module offers.
	Last edited by: danw at: 05/01/2022 23:46:15
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