Epson SQ-2550 User Manual page 237

The column heading "Direction" refers to the direction of signal
flow as viewed from the printer.
"Return" denotes the twisted-pair return, to be connected at
signal ground level. For the interface wiring, be sure to use a
twisted-pair cable for each signal and to complete the
connection on the return side.
All interface conditions are based on TTL level. Both the rise
and the fall times of each signal must be less than 0.2
microseconds.
Data transfer must be carried out by observing the ACKNLG or
BUSY signal. (Data transfer to this printer can be carried out
only after receipt of the ACKNLG signal or when the level of
the BUSY signal is LOW.)
Interface timing
The figure below shows the timing for the parallel interface.
8-12 Technical Specifications