The USBDMX use on a Mac consists of different parts of software that you can find at different places in the machine. First you need the above mentioned driver which is responsible for doing the communications between the device and the mac. This is a system extension which you can find in the system extensions folder. For making use of the new functionality you need to have an Mac application. If you need just snapshots of the memory you can halt the drivers cycle with an accessor call to save some ticks. Remeber that there is the double DMX flow between device and Mac - which is in fact 44*513 bytes*2 per second.
Furthermore in this driver is some functionality present which is not possible to implement somewhere else (at the moment). To set up these functions you need ResEdit to alter two resources in the driver file which define the work of these functions.
The driver itself runs not in your appls heap and not in your time. The driver has its memory and all it's processing done in the system. The driver is loaded when you plug the device into the USB port of your Mac and starts working immediately. In the initial setup the drivers turns on transmitter and receiver and does not copy.
The application has not to deal with any of the drivers' tasks. The application obtains the pointer to the API from the system and may use whatever it needs. The driver will not move the pointers for the ram mirror of the data memories. All control data like startcode is set by fully buffered accessor functions. These functions return to the caller normally within some µsec. If during a transaction a DMX error occurs or you unplug the device the accessor call will take a second to complete cause a DMX error takes its time. The DMX spec defines an error and its following reset to be as least 1 sec long.