Epson 8900 Series - Manual (Page 207)

Communication Interfaces 10-16
Interface Signal Descriptions
The following paragraphs describe the serial interface signals.
Protective Ground
The protective ground (PG) line is connected directly to the power
cord ground to provide protection for the equipment and personnel.
Signal Ground
The signal ground (SG) line is connected to the printer power supply
ground to provide a common reference level for all interface signals
except protective ground.
Transmit Data Signal
The transmit data (TD) line transfers bit-serial data from the printer
to the host via the data communication equipment. During times when
no data appears on the TD line, the line is held in a mark condition.
Receive Data Signal
The receive data (RD) line transfers bit-serial data from the host to
the printer via the data communication equipment. During times
when no data appears on the RD line, the line should be held in a
mark condition.
Request to Send Signal
The printer generates the request to send (RTS) signal to condition
the data communication equipment to transmit data. When the
printer is in the DIRECT CONNECT mode, this control signal is on
at all times following printer power-up and self-test completion.
When the printer is in the MODEM CONNECT mode, the printer
online state holds RTS on and the printer offline state holds RTS
inactive. (The DIRECT CONNECT and MODEM CONNECT
modes are selected during printer setup via the Setup/Test switch.)