Mitsubishi Electric MELSEC-Q Series User Manual page 76

(2) Connection of extension cable
POINT
When connecting an extension base unit to the main base unit with an extension
cable, always plug the OUT side connector of the main base unit and the IN side
connector of the extension base unit with an extension cable. The system will not
operate properly if the extension cable is connected in the form of IN  IN, OUT
OUT or OUT  IN.
When connecting two or more extension base units, plug the OUT side
connector of the first extension base unit and the IN side connector of the second
extension base unit with an extension cable.
(a) To connect an extension cable to the main base unit, remove
the portion under the OUT characters on the base cover with a
tool such as a flat-blade screwdriver (5.5×75, 6×100).
Insert the driver edge into the gap between the base cover and
the section to be removed. Then raise up the driver edge so
that the section will come off.
Make sure not to insert the driver edge until it damages the
connector inside the base cover.
This also applies to a case where an extension cable is
connected to the OUT side connector of the extension base
unit.
(b) To connect the extension cable to the next extension base unit,
remove the seal applied under the IN characters on the base
cover.
Base cover
Seal
42