Epson SQ-2550 User Manual page 182

Installing the board
There are two basic interface board designs, which differ with
respect to how the frame ground (FG) wire is attached. This
slight difference does not affect the operation of the interface in
any way.
1. If the FG wire is not attached to the interface board, attach
the plug end of the wire onto the FG pin located on the
board.
Using Printer Options
Interface Boards
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