Don -

Most of the serial mouse equipment you'll find now comes with the round IBM (sometimes called the PS2) connector, but the pins - number & function, are identical to the ones used in the trapezoidal serial connector. The mouse hasn't changed - only the connector.

You should be able to find the adapter that takes the round connector from the trackball/mouse on one end and has the trapezoidal to connect to your DynaVox on the other. It doesn't do any "converting," it just hard-wires the pins to the right shape to make the connection. It's just like putting a very short extension cord between the light bulb and the wall socket.

A minimally competent technician should be able cut the round connector off the mouse/trackball and put a trapezoidal on in its place.

These adapters were very common when IBM came out with the PS2 series of "PCs" back in ancient times. They may, admittedly, be a little hard to find now, since almost all the PC manufacturers have used the PS2 connector for the last several years on mouse/trackball/joystick stuff. It's still a plain old serial port - just a different shape.

John