Epson EPL-5700 Reference Manual page 97

Negotiation phase: This signal is set high in order to authorize
1284 compatibility. In nibble mode, after the host sets HostBusy
(nAutoFd) high, this signal is set low in order to indicate that the
transmission data is ready.
Reverse data transfer phase: This signal is set low in order to
indicate to a nibble mode host that the transmission data is ready.
Afterwards, this signal serves as nibble data bit 0 and then 4.
Reverse idle phase: Indicates whether or not the data is valid.
ECP mode: This signal is set low to request communications with
the host. Valid in both forward and reverse directions.
nSelectln/IEEE1284active
Compatibility mode: Always low
Negotiation phase: This signal is set high at the same time that
HostBusy is low in order to request 1284 mode.
Reverse data transfer phase: This signal is set high to indicate that
the bus direction is from the printer to the host. This signal is set
low to terminate 1284 mode, setting the bus direction from the
host to the printer.
Reverse idle phase: Same as reverse data transfer phase
ECP mode: Always high. This signal is set low to terminate ECP
mode and return to compatibility mode.
GND
Twisted pair return signal
CG
Connected to the printer chassis. This signal and GND are
connected.
Technical Specifications
A
A-11