Citizen Systems CLP-6401 - Manual (Page 73)

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(2) Ready/Busy system (see the figure)
This is a control system in which the DTR signal is output at Ready
(High)/Busy(Low) level.
Requirements of DTR "High":
･ When the printer is "on line."
･ When the Receive buffer has at least 2K bytes available.
DTR signal is "Low":
･ When the Receive buffer has less than 2K bytes available.
When this condition is detected, the printer keeps the DTR
signal “Low” until the Receive buffer has at least 4K bytes
available.
･ When a printer alarms occurs (i.e. when the printer is switched
to "off line").