Yes, you can use a digital signal on the 3984 to control direction, disable, etc inputs. The digital signal must be from an open collector type device that is capable of sinking a 10mA minimum current and withstanding 12VDC. The 10mA is the minimum value for the open collector device, and is the typical current that is going to be used by the circuit. Anywhere up to between 30mA and 50mA on the open collector device is acceptable to use.
You can use the Red Lion Cub 5 Series digital rate meter. This will provide you with a digital readout of the speed at the motor/gearmotor driveshaft, from the tachometer output on the control. The rate meter is battery powered, and can be programmed to know how many pulses it will receive per unit output and to display speed in RPM. Our tachometer output provides the meter with 12 pulses/motor revolution. To calculate the resolution of the tachometer output with respect to the driveshaft of a gearmotor, multiply the gear ratio by 12.