st3485EC rs485 to uart problem

[Raider E]

Posted on March 12, 2017 at 12:48

Hi Everyone,

I ma using the st3485EC rs485 to uart interface to send/receive data to 485 device. I transmit data with no problems and i can see the device replying back with some strings. However, I am not able to receive anything on the uart rx pin which is pin number one on the chip above. Now I suspect that my connections might be wrong. I am connecting the RE(active low) and DE (active high)togather  to one of the GPIOs, assuming that when i transmit i set that gpio pin to high and when receive set it to low. Even read it somewhere else that this is the most common way of doing it. However, this contradicts the datasheet and i suspect that could be the cause. The data sheet truth table says if DE is low and RE is high the chip goes into shutdown mode. I scoped pin 1 on the chip and its dead nothing i get but on transmitting pin 4 its all good. Anyone has experience with this chip and would be great if you can post the sequence of tuning the enable pins on and off to enable the  receive and send

[raptorhal2]

Posted on March 12, 2017 at 15:20

It's been a while since I did any RS485ing, but I believe the data sheet is correct. Leave the GPIO high for both receive and transmit mode. DE and RE are separate pins so you can use two GPIOs to put the RS485 transceiver in high impedance mode.

Cheers, Hal

[sachin singh]

Posted on August 28, 2017 at 20:44

Hi Mr Raider , i have the exact same problem. I have connected RE and DE together to a GPIO output, Transmit part is working as expected but when i put the RS485 chip in Receive mode by GPIO = Low , Receive Interrupts fire randomly.  Is there any specific order / time constrained to be followed to place the ST3485 chip in receive mode ?