ST4485EB : is it 5V compatible

[Nilav]

Hi,

The ST4485EB IC mentions that it needs a 3v3 supply. How can we ensure that the A and B lines interoperate with other RS485 devices working at a 5V logic level as the datasheet mentions

. 

Should I just provide 5V supply to this chip so that it interoperates? Will that be ok as the datasheet says the max Vcc is rated to be 7V.

Regards,
Nilav 

[Peter BENSCH · Accepted solution]

The RS485 transceivers have two sides:

• the logic interface to the controlling MCU/MPU
• the physical RS485 layer

RS485 operates differentially with low voltage levels and has nothing to do with the supply voltage.

Hope that helps?

Regards
/Peter

[Nilav]

@Peter BENSCH thanks for the response. Then is it that the supply voltage Vcc (supply) will only be responsible for the peak voltage seen on the RO and DI lines interfacing with the MCU (STM32 in this scenario)? 

On the A/B side (which interfaces with the external device i see that the A and B lines are always pulled up to 5V -  

All the schematics show this. Maybe a bit out of context, but would be really helpful why this pull up is required. 

[TDK]

The pullup/pulldown pair is used to give the signal a known state when the chips are not powered. This avoids floating inputs when chips are in a reset state or being powered up. After the chip is powered, these are no longer needed, but of course they're still there.

Lots of RS485 receiver chips can detect open lines and will default to a high state if inputs are not connected.

[Nilav]

Thanks @TDK . Appreciate the succinct response. Makes sense.