Citizen CLP-2001 User Manual page 66

Note: Even if each code is ready for output, the same code cannot be
transmitted twice successively (except when the power is turned
on or the printer is reset from the control panel).
(
2) Ready/Busy system (see figure)
This is a control system in which the DTR is output at Ready (High) or
Busy (Low) level. The DTR, however, is always output at Ready (High)
level except the following:
Requirements of output of DTR 'Busy (Low)':
• When the receive buffer has less than 2K bytes available. When this
condition is detected, the printer keeps a 'Low' level until the receive
buffer has at least 4K bytes available.
Receive buffer size = 32K bytes
Receiving data
X-ON code output
Fig. Buffer in use
6-8
4K bytes
2K bytes
X-OFF code output