Epson 500 Series - Manual (Page 53)

Connecting an External Keyboard
If you are typing for extended periods of time, you may want to
connect an external keyboard to the KB on your computer.
Although the KB and MOUSE ports look the same, they cannot be
used interchangeably. Be sure to plug your keyboard into the KB
port to avoid damaging your system. You can use any keyboard
with a PS/2-compatible connector.
Connecting a Pointing Device
You case use the MOUSE port to connect the trackball or any
PS/2-compatible mouse or pointing device. See Chapter 1 for
instructions. The KB and MOUSE ports cannot be used
interchangeably. Be sure to plug your pointing device into the
MOUSE port.
4-8 Connecting Optional Devices