Below is my RS485 module that interfaces to an ATSAMD21 micro.
The MAX3085 chip is rated 5V while the SAMD21 at 3.3V.
So I had to use level shifters at the input of the MAX3085. Dont ask why I didn’t get a 3.3V chip! (Had lots MAX3085, couldn’t easily find a 3.3V version of the chip with all the other parameters etc)
As you can see from the schematic, I made a mistake and forgot to level shift the DE input which enables the chip on command from the MCU.
The DE input being active high needs 5V to get enabled and it gets about 2.6-2.7V (measured). Not sure if this is enough but the fact is that the system doesn’t transmit while reception is fine.
2.6V active high in a 5V system is on the borderline.
I now need to verify that indeed this is the problem and I am looking at ways to inject a “real” 5V high at he DE input (pin 2,3) without of course damaging the mcu.
Unfortunately everything is SMD and the resistors shown are all part of resistor networks using 4x0603 chip resistors. So I cant just unsolder one of them to isolate the MAX8035 from the MCU.
What if I feed the MAX3085 DE input with 5V via an external -say 4k7- resistor in order to raise the active high voltage? Will this damage the MCU?