Epson TM U200D - B/W Dot-matrix Printer Technical Manual page 153

"H" indicates:
<Printer transmission data>
<Printer reception data>
"L" indicates:
<Printer transmission data>
<Printer reception data>
The transmission data is H = 1, L = 0
Note: This format is used when the UART for RS-232 is connected to the RS-485 driver.
Printer reception data level
RD1 RD2
H L
L H
Printer transmission data level
SD1 SD2
H L
L H
Rev. B
SD1 < SD2
RD1 < RD2
SD1 > SD2
RD1 > RD2
Receiving data line is low level
Receiving data line is high level
Sending data line is low level
Sending data line is high level
TM-U200D/U200PD Technical Manual
Read data
Send data
Appendix A-5