Epson Serial Interface GQ-3500 - Manual (Page 9)

The jumpers at J2
Jumpers at J2 are single position jumpers; they can be either
installed or disconnected. To change the settigs of these jumpers you
must put a jumper plug across the two pins. To disconnect a jumper plug,
simply move the plug so that it
rests
on the left pin only, leaving the
right pin open. (All plugs are in this position when the interface is
delivered from the factory.)
Table. Function so the jumpers
T2
Tl
E L
D M
DM+
SD+
TR+
TR-
Connects DM+ and DM- through a 120 ohm resistor.
Connects RD+ and RD- through
a
120 ohm resistor.
Connects RD- to pin 9
Connects RD+topin 10
Connects DM- to pin 23
Connects DM+ to pin 24
Connects SD+topin 12
Connects TR+ to pin 25
Connects TR-to pin 13
Communications Protocols
To achieve the maximum printing speed, the computer should send
characters to the printer faster than the printer can print them. This
ensures that the printer has to stop and wait for more
information. But if you continuously send information to the printer
faster than it can print, sooner or later the printer’s buffer will
overflow and information will be lost. A communications protocol keeps
the computer from sending more information than the printer can
handle. *
The GQ-35CN7's serial interface can use three different kinds of
communications protocols These are described in detail below.
The ETX/ACK protocol
The ETX/ACK protocol uses a special pair of control codes, ETX
(ASCII 3) and ACK (ASCII 6). The system is very simple: whenever
the GQ-3500 gets an ETX out of its input buffer it sends an ACK back to
the computer. When the computer receives the
ACK it knows it is safe
to send more information to the printer without fear of overflowing the
buffer.
8 Serial Interface