Citizen CBM-231 User Manual page 27

(1) RD
This is a serial receive data signal. When there is a framing error, overrun error, or parity error, that data
is printed as "?."
(2) DTR
Write the data or a command when this signal is ready. If you write while it is busy, the data will be
ignored, resulting in an overrun error. The data can be written into the input buffer even during printing.
A BUSY signal is also issued at power-on, during test printing, in the on-line mode, or upon occurrence
of reset.
(3) TD
When the remaining capacity of the input buffer on the printer side comes to 10 bytes(at 72 bytes) or 128
bytes(at 4K bytes) while receiving the data, DC3(13H), a data reception impossible signal, is output.
When the remaining capacity of the input buffer comes to over 20 bytes(at 72 bytes) or 256 bytes(at 4K
bytes)DC1(11H), a data reception possible signal is output to the host side.
When transmitting the status information, if DTR/DSR control has been selected, the data will be
transmitted after confirming that DSR is a space.
If DTR/DSR control has not been selected, the data will be transmitted, ignoring DSR.
(4) FG
This is a Frame ground.
(5) GND
This is a common ground on the circuits.
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CBM-230/231 User's Manual
CITIZEN