Citizen Systems Citizen CLP 6001 CLP-6001 - Manual (Page 67)

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6.2.3 RS-232C loopback test
After connector wiring as shown in the figure, the test mode is turned on.
The printer will receive data that has been transmitted by printer itself and the
test of receiving and transmitting data will be performed.
RS-232C test
Fig. Loopback test