Epson LQ-2550 User Manual page 192

Installing the board
There are two basic types of interface board designs, which slightly
change the way they are installed in the printer. (The frame ground wire
is attached for one type and not attached for the other.) This difference
does not affect the operation of the interface in any way. Check to see
which type of interface board you have and then follow the instructions
for that type of board.
FG wire not attached
Use the CG screw to attach the round end of the FG (frame ground)
1.
wire to the main board and position the other end as shown.
Using Printer Options
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