This paper reports the PC implementation of a real-time simulator for testing protective relays, based on the widely used EMTP-ATP software. The proposed simulator was implemented using the GNU/Linux OS with a real-time kernel. In order to generate the waveforms corresponding to simulated voltages and currents, a PCI card was used. This card also includes digital I/O interface. Via foreign models programmed in standard C, ATP was recompiled to include waveform generation at each simulation time step and digital I/O. Additionally, an IEC-61850 open source library was used, in order to use Sampled Values and GOOSE protocols. The resulting tool is a real-time simulator that can interact with protective relays by means of HiL tests. The performance of the simulator was analyzed via an interaction with an actual relay.