Epson Serial Interface GQ-3500 - Manual (Page 8)

The jumpers at Jl
The jumpers at J1 are two position jumpers. The small plugs can be
moved from the left two wires to the right two wires to change the
settings.
Table 3 describes the functions of the jumpers at Jl. When
delivered from the factory, all jumpers are in the left position.
Table3. Functions of the jumpers at J1
Serial Interface
7