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Goals and Symptoms
In a Modbus master-slave network, legacy meter responds only to every other request and ignore the others.
Causes and Fixes
In Modbus RTU communication topology, messages start with a silent interval of AT LEAST 3.5 characters. This is most easily implemented as a multiple of character times at the baud rate that is being used on the network. In legacy Power Measurement meters, depending on the network, the silent interval may exceed 3.5 characters. On a healthy network with baud rate 9600 this gap should be equal to ~3ms, which is too fast for the legacy devices to catch. On a newer network like this any message sent will hit the silent interval on the 3720, so the meter will not respond. However, the following request will be received after enough silent duration and the meter will respond. This will carry on so the meter will only respond to every other request.
Adjusting the RTS (Request To Send) interval greater than 100ms (or longer) on the master side will enable the meter to catch all the request packages sent to the network. This adjustment will manipulate the master to hold 100 ms in between every request which will give enough time to the legacy meters.
Last Revised: April 10, 2008
Applies To: 3720 ACM
Original Author: KG
All content © 1992-2008 Schneider Electric
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