Epson TM-T20III Technical Reference Manual page 88

Functions of each connector pin
Signal
Pin no.
name direction
1 FG -
2 TXD Output
3 RXD Input
4 RTS Output
6 DSR Input
7 SG -
20 DTR Output
25 INT Input
Signal
Frame ground
Transmission data
Reception data
Equivalent to DTR signal (pin 20)
This signal indicates whether the host computer can receive data.
SPACE indicates that the host computer can receive data. MARK indicates that
the host computer cannot receive data.
When DTR/DSR control is selected, the printer transmits data after confirming
this signal. (except if transmitted using some ESC/POS commands).
When XON/XOFF control is selected, the printer does not check this signal.
When you use this signal as the printer's reset signal, the printer is reset when
the signal remains MARK for a pulse width of 1 ms or more.
Signal ground
1) When DTR/DSR control is selected, this signal indicates whether the printer is
BUSY.
•SPACE status
Indicates that the printer is ready to receive data.
•MARK status
Indicates that the printer is BUSY. Select the BUSY condition in the
software setting mode.
2) When XON/XOFF control is selected, this signal indicates that the printer is
properly connected and ready to receive data from the host.
•SPACE status
The signal indicates that the printer is properly connected and ready to
receive data from the host.
The signal is always SPACE, except in the following cases:
•During the period from when power is turned on to when the printer is
ready to receive data.
•While the self-test is in progress.
The printer is reset if the signal remains at SPACE for a pulse width of 1 ms or
more.
Function
Appendix
88