RS485 bus voltage ? can yo mix 5v and 3/3v transceivers ?

[Redguy]

Hi Guys,

I have used the RS485 bus before.. but that was between industrial devices etc.. or connected to a bought RS485 to RS232 adapter plugged into a RS232 port on a pc.. But this time it's for my home domoticz setup..

I don't want to keep plugging in usb converters into my rpi3 for everything so i added some extra serial interfaces using the SC16IS752. It's working now. So now i want to use 1 serial for a RS485 bus to run between my RPI3 and a bunch of MySensor nodes connected though a wired RS485 bus. The other is meant to connect to my Schneider PM9C power monitoring device, which uses a Modbus interface (Also RS485)

So i started to grab an Max485, and then realized it's a 5V device. easily fixed.. Use some level converters or look for a 3.3v transceiver.. But that;s the local interface..



But how about the RS485 bus itself ? It's a differential bus, it has no pull-ups, jus tthe 120 ohm terminators on each end. But what about about the bus voltages ? I dont see anything about voltage pumps etc like used on the max232 to convert the signals to the RS232 spec voltages (okay close enough)

Can you just mix RS485 devices on the bus even though their transceivers run on different supply voltages ?

Looking at the RS485 specs i googled, i think it should work. it all lies withing the voltage ranges specified for the bus. Anyone with some more insight to this ?

[Toulon7559]

RS485 as specification prescribes the signal characteristics at the bus.
Therefore, devices at the RS485-bus can generally be of mixed design -IF- at the bus-side they meet that presciption.

The other side of such device is a different story.
In other words, if you connect the interface-device to Raspberry's GPIO, the GPIO-side of the intdrface-device has to be compatible with GPIO, which is 3V3!
As example, the Linksprite RS485-shield for Raspberry applies an IC type MAX481CSA with VCC=5V at pin 8 and with 3V3 at pins 2 and 3.