I have a new grblHAL setup I'm testing. I am using the Universal GCode Sender running on a Windows laptop, connected to an Arduino Due and selecing the cmcgrath_rev3_map pin map. The grblHAL Arduino is not yet connected to an actual machine. I am checking its performance using an oscilloscope to monitor the signals.
When I start sending the file, I see the dir and step signals coming out of the Arduino as expected on pins 5 and 2 as noted in the cmcgrath pin map. I need to use the FEED HOLD and CYCLE START signals to make temporary halts. I have connected switch signals to Arduino pins 24 and 23 as specified in the cmcgrath pin map. I have also uncommented the following lines in the cmcgrath_rev3_map.h:

#define FEED_HOLD_PORT PIOA
#define FEED_HOLD_PIN 24
#define CYCLE_START_PORT PIOA
#define CYCLE_START_PIN 23

My oscilloscope is connected to FEED HOLD, pin 24, and the step signal. When the FEED HOLD signal goes to ground it has no effect on the step pulses, they continue on.
Am I missing some other piece of enabling the FEED HOLD function? Help!