Max485 3.3v / 5v, not sure

[saburo]

Hello, I have been reading datasheets and many threads about the MAX485 and there is still one thing that is not totally clear to me.

The datasheet states that MAX485 should run at 5v and indeed on my breadboard I was able to power it at 5V and have it communicate successfully with an FTDI cable connected to my PC (using DTR signal to drive the R/W).

But then I tried to power it at 3.3V and it also worked.
My fear, then was that the UART logic signals were not compatible with the 3.3V logic, but I guessed (probably wrongly, please correct me here) that if I was powering the chip with a 3.3V vcc the uart signals could normally not exceed 3.3V, so they were supposed to be "safe" to be connected to a 3.3V uart.

Unfortunately I don't have an oscilloscope, so I cannot check the values.

I am now powering my interface from the 3.3V rail of a beaglebone, which also has a 3.3V uart and since some days it is successfully communicating with other devices on the 485 bus.
My beaglebone seems to survive well.

So, according to my tests the MAX485 should be able to work either at 3.3V or at 5V, but that does not really make sense with the datasheets I am afraid.

What could be the problem at 3.3V? The maximum amplitude of the differential signal (A-B)? If so I am not getting issues due to the fact that I am using a cable which is few meters long?

Sorry for the "noob" questions, highly appreciate any eventual feedback.

Francesco

 

[CMOS]

Standard operating voltage of MAX485 as per its datasheet is 5V. With 3.3V the differential output voltage will be lower which will affect the operation with longer cables. Also proper functioning of the chip is not guaranteed at anything less than 4.75V.

 

[saburo]

Thanks CMOS,
I somehow feared that.
I found that there are driver for 3.3V operations and others, obviously for 5V... was not able to find a "flexible" one that would support automatically the two.
Do you know if such chip exists?
If not, what's the best solution? Adding a logic level converter with 5V on the MAX485 side and Vx on the other side? Should work, I guess even when Vx=5V.

Btw in my tests I accidentally discovered (and I was quite surprised) that in read mode I get data on the uart rx even if vcc is not connected (the MAX485 is not powered).
I was quite shocked to see data incoming on my terminal when I had the Vcc cable in my hand, not connected!
That made me wonder : how can the uart rx signal be affected by the Vcc if I get data even when the chip is not powered?!

Uhm... guess I have to get myself an oscilloscope after all :/