Epson 8900 Series - Manual (Page 201)

Communication Interfaces 10-10
Paper-Empty Signal
The paper-empty (PE) signal from the printer indicates that the paper
supply is empty. The printer activates this signal in combination with
the fault signal to indicate the paper-empty condition to the host.
Select Signal
The select (SLCT) signal from the printer indicates to the host that
the printer is online and ready to receive data. The completion of
either a normal power-up operation or a host-generated initialization
cycle activates SLCT.
Verify Signal
The verify signal confirms proper connection of the parallel interface
cable between the host and the printer. The verify in (VFIN) and verify
out (VFOUT) pins are connected together inside the printer to form a
loopback circuit. The host sends VFIN to the printer and, if the cable
is connected properly, the printer returns the signal as VFOUT.
Ground Signals
The logic ground (LGND) connects to the printer power supply logic
ground to provide a common reference point for all interface signals.
The chassis ground (CGND) connects to the ac power cord safety
ground to provide protection for the printer and personnel.
Parallel Interface Communication Protocol
The parallel interface communication protocol is a set of conventions
that ensures compatibility between the printer and the host computer.
The following paragraphs describe these conventions.
Data-Transfer Timing
Because the parallel interface transfers all bits of a character
simultaneously, correct timing is important to prevent data loss or
corruption. When the host senses that BUSY is inactive (low), the
host can initiate a character-transfer cycle on the interface.