There is a building in which there are 5 floors. Each of them contains about 20 rooms (so there are about 100 rooms). I had to develope a small control device that can communicate through a bus cable.
I choosed the RS-485 interface. The transceiver is the LTC2856-2 IC from Linear Technologies.
The controller I've designed has a DC supply input (12-35V DC). It is used for the complete logical part power supply through a DC-DC converter and a LDO regulator. And the input supply is used as the controller output voltage through some MOSFETS.
The maximum current a module can supply (and so also sink from the power supply) is 3A (not dependent on the DC voltage). That's why I have to use a separate power supply for each of the devices. (Too much current for more devices connected together and the longer the distance between the modules, the thickest the wire I would need for small voltage drop).
The problem is with the RS-485 interface. The datasheet says that the common mode voltage on the RS-485 lines (A and B) has to be between -7 and +12 VDC. How can I achieve/ensure this, if I use separate power supplies for each of the modules? I think then they are just like batteries or a simple transformer, floating supplies.
The only connection between the modules should be the bus-cable, which is a shielded 2-wire twisted-pair cable.
Can anyone help me with this?
Thank you for your help in advance.