Hello,

I have a design using the UART of a PIC24FJ128GU406 connected to a MAX3485 to transmit data over RS485.
It's a very simple design with the UART of the microcontroller connected directly to the RO and DI pins of the MAX3485, a 120R terminating resistor, some TVS diodes and electronic fuses for additional field protection:



(I know the schematic says SP3485, but the chip on my board is the MAX3485 as there was no stock of the original at the moment of ordering).

The problem I am having is that I can send (Tx) data, but I cannot receive (Rx) data, so I hooked up the scope to have a look at the signals.

Here is the measurement of the Tx data (0xAA for simplicity) on the DI pin:




Here is the measurement of the Rx data (0x55 for simplicity) on the RO pin:



Notice that the amplitude of the signal is very small. Merely 500mV, when it should be 3.3V.
I have tested this on 3 different (newly assembled boards) and the result is the same.
I dug through the MAX3485 datasheet thoroughly to check for design flaws but cannot seem to find the issue.

Any idea why could this be happening?

Many thanks in advanced for the help!
Lewis Flavell

 

For anyone wondering, this has been solved and had nothing to do with the hardware.

The pin that controls the UART Rx was also set as a digital output in the firmware, thus causing it to always be low.