Specifications

DTR negated means it is a negative voltage. A voltage less than zero volts
applied to DTR will turn the FireLine off by putting it into LPM.
To assert the DTR signal (turn the modem on), the DTR line must be high (>
3V). DTR is an input to an RS-232 transceiver IC. It is ESD protected, and
is safe for voltages from –20V to +20V input.