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26th February 2016, 12:04 PM
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Join Date: Apr 2013
Re: Modbus RTU Half Duplex

As you have asked about the Modbus RTU and Half Duplex and their uses, I am giving you information about it

Modbus is a "master/slave" communications protocol protocol.
There is one Master, which usually takes the form of datalogger software or a SCADA program running on a computer.
There can be up to 127 slave devices on one multi-drop RS-485 serial bus. The Master typically sends a command to one of the slaves and waits for its response.
The RS-485 bus used by WattNode Modbus energy meters (and most other slave devices) is referred to as a "two wire" half-duplex bus (vs. "four wire" full-duplex).
A half-duplex, two-wire bus requires fewer signal conductors, but only allows one device to transmit at any given time.
Although the RS-485 half-duplex bus standard is commonly referred to as "two-wire", keep in mind that a third wire is also needed for a zero volt reference (i.e. signal common).
To avoid introducing ground loops, the signal common wire should only be grounded to earth on one end (usually at the master).

A slave may only transmit in response to a command from the Master that is specifically addressed to that slaveā€”the address is determined by the WattNode's DIP switch settings.
To avoid bus contention, every slave must be assigned a unique address by the installer.
DIP switch positions 1 to 7 set the slave address from 1 to 127, and position 8 determines the baud rate (ON=19200, OFF=9600).
As shipped from the factory, the address is set to zero (not a valid address) and the WattNode Modbus LED will turn red to alert the installer that the address needs to be assigned.
If the WattNode meter is the only slave device on the network then flip position 1 to the ON position to assign a slave address of 1.
If there are more than one WattNode then the other units must be assigned a different address (e.g. leave position 1 OFF and set position 2 ON to set the next unit's address to 2).
Each subsequent switch position has a weight twice that of the preceding one (e.g. position 1 has a weight of 1, position 2 has a weight of 2, position 3 has a weight of 4, position 4 has a weight of 8 etc.).


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