Driver Enable of PMOD RS-485

[Arthur3598]

Hi,

I am using the PMOD RS485 module with an FPGA and it seems like I cannot set the pin DE (Driver Enable) always high in half-duplex. It is sort of preventing me from acquiring signals through RXD. 

Is it because Y and A are connected together, as well as Z and B? Should I keep DE low unless I want to send data from the PMOD module?

Thanks in advance.

[JColvin]

Hi @Arthur3598,

Correct, because of the half-duplex nature the receiving and transmitting lines are tied together through a termination resistor. If a Driver Enable is set high on one node, some other node will not be able to transmit data, so you would only want to keep DE high on the Pmod RS485 when you wish to transmit data and then either put DE low or into tri-state (as there is a pull-down resistor implemented on the Pmod) while not actively transmitting data.

The ADM2582E datasheet (https://www.analog.com/media/en/technical-documentation/data-sheets/ADM2582E_2587E.pdf) doesn't do a great job at explaining this; I personally found the AN-960 (https://www.analog.com/en/app-notes/an-960.html?doc=ADM2582E_2587E.pdf) to be more helpful for using the module in half-duplex.

Let me know if you have any questions.

Thanks,
JColvin