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5/1/2012 4:24 PM | |
Joined: 1/17/2007 Last visit: 8/21/2024 Posts: 1545 Rating: (537)
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I am not sure that I fully understand your question. So let me try to advise you as to what you need to be aware of when implementing multiple slave modbus networks. Firstly, let us start with how to physically connect the slave devices. The best way is to use a multi-drop network. This is where all devices are connection in a bus topology with the connecting cables looping-in and looping-out of each slave device. To use this setup, you will need to use the RS485 electrical interface. This is available as standard for all serial CP's supported by Siemens. RS485 limits the total ammount of devices on the same bus to 32 (1 master and 31 slaves). The slave that is last on the bus (furthest away from the master) will require a terminating resistor to be fitted (120Ohms). Secondly, you must ensure that all slave devices are assigned a unique slave address. This is usually performed via the configuration software for the slave devices, or via hardware DIP-switches. In addition, all devices on the same bus, must be configured to the same serial interface settings (e.g. baud rate, parity, stop bits etc.). Thirdly, you would then load in the modbus master protocol driver for the CP card. This is available for CP341 and CP441-2 only. See the modbus general description sticky tip for details of where to obtain these drivers. Forthly, you would then need to get yourself an example project for a modbus master. See this thread. Then use this as a base to develop your PLC programming to make you modbus interface. Hope this helps. |
Programming today is the race between software engineers building bigger and better idiot proof programs, and the universe producing bigger and better idiots. |
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