Serial Port; Serial Port Protocol; Figure 18 Serial Port Flow Control Using Dtr - TransAct Ithaca 8000 Programmer's Manual

Communications

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
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255 bytes of data after it indicates that it is not ready. Figure 18 Serial Port Flow
Control Using DTR illustrates how the Ready/Busy protocol works, and Figure 19
XON/XOFF Serial Port Flow Control illustrates how the XON/XOFF protocol works.
Communications
Port
Serial
Data In
Data
Inquire
Serial
Response
Data Out
(Not Used for
Flow Control)
Clear
DTR
Set
DTR
RTS
RTS = Request to Send

Figure 18 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.
100-06908
ENQ
Data Data
Proc.
ENQ Response
Buffer Getting Full
Buffer Getting Empty
Select Key
DTR = Data Terminal Ready
Rev D
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Ithaca 8000 Programmer's Guide
Printer Control
Data
Print
Buffer
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Software
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