Using MAX13487 on a 3.3v UART?

Hello Sebastian,

the MAX13487 is a 5V part and the output voltage on the RO pin will exceed 3.3V, Vol(min) = Vcc-1.5V. ADI does not have an auto-direction RS-485 transceiver which can operate at 3.3V.

If you want to use MAX13487 then you would need a logic level translator such as MAX13003-5 family MAX13000E DS.qxp (analog.com). If you are you planning to add galvanic isolation between the RS485 and the MCU, then the digital isolator can also do the voltage level translation, and D-ISO products  are available with different channel options Digital Isolator | Analog Devices 

If you just want a single chip RS485 solution without any isolation, then some of ADI's competitors have auto direction RS-485 with 3.3V capability.

thanks

Sean.

Thank you for your reply Sean, i ended up making the isolated suggestion some day ago. good to have it confirmed that it should work

i found the schematics here: MAX13487E EV KIT (analog.com)

one thing i thought about is the pull up/down on the rs485 A and B, why are the resistors this low value? (220Ohm, R6, R8 in the MAX13487E EV KIT link) 


your soloution to use Digital Isolators is a much better idea for my project than the solution i have used, i found this on your link: ADuM320N/ADuM321N (Rev.C) (analog.com), looks like i just need two 0.1uF capacitors to get this working! then i can save alot of space. so this will be my future choice even if i dont need the isolation.



Regards
Sebastian

Hello Sebastian - the digital isolators are a lot more flexible solution than the opto couplers shown on the original evkit.  The MAX13487 requires these failsafe resistors as described in this app note Guidelines for Proper Wiring of an RS-485 (TIA/EIA-485-A) Network | Analog Devices  The value selected will be influenced by the data rate and number of node on the network - with an auto direction transceiver you want the line to drive high and release quickly, so a lower value resistor makes any RC spec less. 

Hi Sean, i missed your reply, sorry about that, just saw it
im really glad you showed me the Digital isolators! i will have alot of use for this :)
just thinking of the GND1 and GND2, can this be the same if im only gonna use this as an level shifter?

if i want isolated i think i can put an 0 ohm resistor between ground planes to split this up in 2 different ground planes? or how should you have done this?



the picture is what i could understand of this schematic, this time im using it between a ATmega32M1 UART to an Orange Pi 5+ UART

Hi Sean, i missed your reply, sorry about that, just saw it
im really glad you showed me the Digital isolators! i will have alot of use for this :)
just thinking of the GND1 and GND2, can this be the same if im only gonna use this as an level shifter?

if i want isolated i think i can put an 0 ohm resistor between ground planes to split this up in 2 different ground planes? or how should you have done this?



the picture is what i could understand of this schematic, this time im using it between a ATmega32M1 UART to an Orange Pi 5+ UART

Hi Sebbe, if you are designing for galvanic isolation and to meet a specific safety standard, then you need to consider factors such as creepage and clearance and with two isolated power supplies having two separate grounds (GND1 and GND2),  the area beneath the isolator chip should be left open to avoid any galvanic connection between the two sides. You can see an example of such a layout in the attached evkit picture below. If you plan to use the D-ISO chip as a level shifter only then you can have a common ground. I am not too familiar with the Orange Pi5+ but if you are trying to future-proof your design you could use a method such as you describe to link or open the two separate ground planes.