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S7-200 communication with ModBus slaver

7/17/2009 9:38 PM	Rate (0)
Tomy Master Platinum Member Joined: 5/4/2009 Last visit: 1/1/2026 Posts: 916 Rating: (40)	Hello every body..... I need help.... I am trying to connect the S7-200 with a Modbus slave ( non-SIEMENS AC motor drive) using the Modbus library instructions. In the S7-200 communication port we have the pin 3 for RS-485 signal A, pin 8 is RS-485 signal B and pin 5 is logic common. In the Modbus slaver we have pin 3 is logic common, pin 4 RS-485 signal(-) and pin 5 is RS-485 signal (+). I want to know what should I connect with what to make connection successfull? Regards. Tomy.
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7/19/2009 11:58 PM	Rate (1)
Darius Platinum Member Joined: 9/28/2005 Last visit: 12/9/2025 Posts: 2957 Rating: (243)	Hello, Connect Data A to Data "+" and Data B to Data "-". Common pins also connect together.
	Best regards, Darius
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7/20/2009 12:08 AM	Rate (0)
Tomy Master Platinum Member Joined: 5/4/2009 Last visit: 1/1/2026 Posts: 916 Rating: (40)	Dear Darius.... I am really greatfull, thank you Regards. Tomy.
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7/28/2009 10:59 AM	Rate (0)
dezile Posts: 11 Rating: (0)	Why we use common pins for RS485 communication?
dezile Posts: 11 Rating: (0)
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7/28/2009 11:17 AM	Rate (0)
dchartier Grandmaster Joined: 9/27/2006 Last visit: 1/16/2026 Posts: 12481 Rating: (2722)	Hello dezile; For RS-485 connections, 3 wires are required; Data+, Data-, and Signal Common. Hope this helps, Daniel Chartier

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7/31/2009 11:26 PM	Rate (0)
Tomy Master Platinum Member Joined: 5/4/2009 Last visit: 1/1/2026 Posts: 916 Rating: (40)	Dear Dezile.... It is important to use the common pin inorder to unite the reference. Regards. Tomy the Master.
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