Casio UP-250 Technical Manual page 148

"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
SD1 > SD2
RD1 > RD2
Receiving data line is at the LOW level
Receiving data line is at the HIGH level
Sending data line is at the LOW level
Sending data line is at the HIGH level
— 139
Read data
Send data
—