Hi.

I'm relatively new to MODBUS.
I'm creating an interface for my HVAC system in my house.

I'm planning to control it from a raspberry pi, where i have all my sensor data. All scripts etc are good to go, but i need help wiring up the communication to the HVAC unit. (modbus RTU)

I have a usb to RS485 converter from my raspberry pi.

But the VCC out of the HVAC unit is 12V and the VCC out of the USB port is something else.
Could i just leave out the VCC since raspberry pi is self-powered?

Check out this:
https://drive.google.com/file/d/0B3Eq5NUU1ZDPUXhERl9TXzJjQWc/view?usp=sharing

I need help where the question mark is.

Thank you!

 

Hi,

You do not need to connect VCC. In theory you only need to connect the A and B lines since RS485 is a differential bus but it is always preferred to also connect GND.

 

I have just been doing RS232 Modbust to a Pi so this caught my eye. There is no VCC associated with the RS485. All you should need for half duplex are 3 wires a + a - and a ground. I have some USB to 485 converters on order so I'll be doing multidrop soon. The Pi with Python makes a good rapid development HMI. And it makes an inexpensive and flexible bridge between real Ethernet networks, (not the expensive and perverted automation types) and the automation world.

Regards
cww