Wire sensors to usb/RS-485 converter


I want to communicate with two forwire sensors. Can I use a breadboard and jumper wires for connection?

What are the sensors you are planning to use?

Modbus perssure sensors.

Which ones?
We may have to look in the datasheets to find an answer


What can I use to do the connections? Jumper wires? Breadboard?

Up to you, the dongles have screw terminals, so use that.
Breadboard and jumper wires for testing. When it works use screw terminals and wires or even better solder.

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Hi @dohan,
I have just googled at little bit and there are youtube videos and diy blogs on how to do that.
I have never worked with modbus and/or RS485 before so I won’t be much more help here.
The schematic above is pretty self explanatory.

You only have to connect Pin 3, 4, and 5 with the usb to rs485 converter. Additional you have to supply betwen pin 2 and pin 5 with 6 to 24 VDC.

With Pin 3,4,5 you mean “A” and “B” and “GND”? What about Pin 1= “+” ?

Yes. Pin 1 with the name: 4-20mA seems to be a analoug out of the pressure. You can leave it open.


Don’t apply the 24V power to the USB/485 converter. It’ll be powered from the USB plug and will not want external power, you’ll probably damage it.

You need to check if your 4-wire USB/485 converter will run in 2-wire mode to suit your sensors. If not, use the 2-wire USB converter.

You should connect the power to the + - pins of the sensors of course.

Unless you have long cable runs using screened cable, you should not need to connect the Gnd pin of the converter.

You should use twisted pair wires for the A B wires, taken from a bit of UTP cat5 cable perhaps.

You may need to add a terminator resistor or two unless your cable run is very short.

Remove the power supply, all the power will come from the USB dongle.
The resistor is in the right place. You may need it, you may not, Experiment!
Use Cat5 Ethernet cable if you can
The length really depends on your USB dongle. 10-15m maybe.
Read your dongle specs.

Why remove the power supply? The sensors needs to be powered with 6-24VDC…:thinking:
How do I realize that I need the resistor? Additional noise? 120ohm ok?
I dont have any ethernet inputs/outputs… or do you mean just the wires from it?

Keep the power supply but make sure you don’t connect it back to the dongle or you will fry it.
For the resistor, you need to test. Test with it, test without. Choose what works.


Ok, so this wiring-solution will work?

Yes to the ground on the usb dongle