2-wire vs. 3-wire vs. 4-wire Modbus

[Dave Loucks]

Don't forget that most Modbus devices are 2-wire half duplex, but that they need a 3-wire connection!

Why is that?

Consider an EIA-485 system with only two wires connecting the nodes together.  The node transmitting places a voltage on the, for example, non-inverting wire.  The diagram below is simplified (485 is a bipolar output voltage, therefore four transistors in an H-bridge are needed for each line.  Here only one on each is shown). 

For current to flow, a complete circuit must exist from the point the voltage is placed on the non-inverting line, until it returns to the node placing the voltage on the line.



In actual systems, there will be a return path through the biasing resistors (if connected), but typical wave traces show offsets to ground over time.  This can be due to line-to-ground capacitances charging and discharging.



A better system will include the 3rd (common) wire.



What if you only have a single twisted pair?  It is acceptable to use the shield as the common.  Just remember to only ground the shield at one location, but to connect the shield to the common at every device.  An example is shown below: