to answer:
You need to set your modbus in brains correctly: Please READ the "Brains User's Guide" under member docs....
this ASSUMES (per the way you asked your questions, that you are using the OLD style serial modbus).
so under ports/pins, make sure you have the Enable Modbus support checked, (but NOT the enable plugin serial modbus if your wanting to use the old style serial mb). Make sure you set up your, modbus config parameters correclty and test them with the modbus test dialog.
in Brians, ASSUMING again, that in the old style modbus, your using the first "row" of outputs, *which are discrete* not word.
your rung would look like this: the important pick is how you pick your terminator for serial modbus.
when you set your terminator: you want to pick "output" check box, local address "0" (assuming your estop is the FIRST in the register bit map). tick the User NON-plugin serial modbus radio button, and tick "0" for the config.
here on my comp, when I push the Estop button, then the Non-serial MB term goes to 1 also.
scott