Polling Modbus slaves from Modbus Master / Profibus Slave Gateway
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prnelson
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Thursday 11 January 2007 10:38:51 am |
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Polling Modbus slaves from Modbus Master / Profibus Slave Gateway
I need to setup a network using Modbus RS485. It will consist of multiple modbus slave nodes - small controllers. All these nodes are to be connected via a gateway to a PLC with Profibus DP I/O capability.
I need to be able to poll the slaves for data as well as occasionally write to a few registers to enable / disable an algorithm or update a variable.
Is there a way to automate the polling mechanism in the gateway, or will I have to do the polling of nodes one by one using logic in the PLC?
thanks,
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jsanders
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Thursday 11 January 2007 3:38:43 pm |
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RE:Polling Modbus slaves from Modbus Master / Profibus Slave Gateway
To get data from or send data to Modbus Slaves, you will need to create a poll list, or set of commands, in the ProLinx module configuration. Once that is done, the module will issue whatever read and write comamnds you create, up to 100 commands per Modbus port, without any logic needed on the Profibus side.
Any data read in from the Slave(s) by Modbus Read commands will need to be configured to store that data in the registers assigned for Profibus Inputs. And data to be written out by Modbus Write commands will have to be taken from the area of the module's memory assigned as Profibus Outputs. This is how the two protocols transfer data between them.
The only logic required in the Profibus PLC will be to monitor the Profibus Inputs and set up the Profibus Outputs.
Bear in mind, the Profibus driver has limited data transfer ability, a maximum of 122 words of data In, a maximum of 122 words of data out, with the combined total of the two not more than 200 words in total. The Modbus driver could use all 4000 registers of User Memory, if it wanted to; and this is the case when the other protocol is something other than Profibus. But Profibus Inputs can be put only in module memory addresses 0-121 and Profibus Outputs can be put only in module memory addresses 200-321; so, this limits the addresses that can be used to pass data on the Modbus network.
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