iTherm
®
280 Programmer’s Guide Communications
28-07764 Rev C Page 309
Serial Port
Serial Port Protocol
The serial port supports two flow control standards, XON/XOFF and Ready/Busy
(sometimes called Data Terminal Ready (DTR) or hardware handshake).
When Ready/Busy flow control is selected, the printer can be configured to use DTR,
Request to Send (RTS), or both for flow control. If only DTR is selected for flow control,
RTS will indicate the cover is open or the printer has faulted. The following discussion
assumes the DTR is being used for flow control.
The Ready/Busy protocol generally uses the DTR signal to indicate to the host computer
that the printer is not ready to accept data. The host should stop sending data to the
printer as soon as possible. Because the host may not notice the DTR signal until it has
transmitted several bytes of data to the printer, the printer continues to except up to
255
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bytes of data after it indicates that it is not ready. Figure 26 Serial Port Flow
Control Using DTR illustrates how the Ready/Busy protocol works, and Figure 27
XON/XOFF Serial Port Flow Control illustrates how the XON/XOFF protocol works.
Serial
Communications
Port
Data
Buffer Getting Empty
Select Key
Printer Control
Software
Print
Buffer
RTS = Request to Send DTR = Data Terminal Ready
Data In
Serial
Data Out
(Not Used for
Flow Control)
DTR
RTS
DTR
Clear
Set
Inquire
Response
40 - 8K
Data
Buffer Getting Full
Data
ENQ Response
Data
ENQ
Proc.
Figure 26 Serial Port Flow Control Using DTR
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The buffer always signals it is full before it overflows. The size of the reserve depends on the
buffer size selected. It is always at least 255 bytes.
