EPI’s medical device customer wanted to design a radiology analysis system capable of using either a mouse or a trackball for operation. The trackball needed to be in an enclosure, tethered by a 12-foot cable, while the mouse, if used, had to be restricted to a worktable. If the mouse was not attached, the trackball had to respond to take the place of the mouse. This particular objective meant that the interface from the keypad to the host must be electrically identical to the mouse and support the same communication protocol.
Solution: EPI engineers reasoned that, due to the physical locations of the trackball and the mouse, the implementation required the use of three low-cost microprocessors. Microprocessor #1 (P1) communicates with the mouse and sends pertinent information via serial connection to microprocessor #2 (P2). P2 is located in the keypad and interfaces with the trackball. If allowed, it converts trackball movements to mouse information. P2 also interacts with microprocessor #3 (P3). In turn, P3 interfaces with the host computer and converts information received from P2 to standard mouse signals.  If the mouse is present, movement is detected by P1, sent to P2 and passed to P3 for final transmission to the host. Commands from the host are converted to serial signals and, through a series of interactions among all microprocessors, sent to P1. P1 converts the appropriate command to mouse signals and communicates with the mouse to elicit operation. If the mouse is not present, P1 sets a signal level that informs P2. In this situation, P2 interprets the trackball signals, and makes the appropriate response to P3, which passes the information back to the host. Commands from the host either change the mode in or force responses from the trackball processor.
Results: EPI successfully produces an MRI Scan Controller meeting all of its customer’s design objectives, including the flexibility of using a trackball or a mouse during operation.