Epson 386/25 - Manual (Page 115)

Setting the Jumpers
Turn the computer so the back panel faces left and the front
panel faces right. The illustration below shows the locations of
the jumpers.
A jumper’s setting is determined by where the jumper is placed
on the pins. The jumper either connects pin A and the middle
pin (position A) or connects pin B and the middle pin (position
B), as shown below.
position A position B
Installing and Removing Options 5-7