Serial Port; Serial Port Protocol; Figure 36 Serial Port Flow Control Using Dtr - TransAct POSjet 1500 Programmer's Manual

Programmer' s Guide

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
bytes of data after it indicates that it is not ready. Figure 36 Serial Port Flow Control Using DTR illustrates how the
Ready/Busy protocol works, and Figure 37 XON/XOFF Serial Port Flow Control illustrates how the XON/XOFF
protocol works.
Communications
Port
Serial
Data In
Inquire
Serial
Response
Data Out
(Not Used for
Flow Control)
DTR
RTS
RTS = Request to Send
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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.
Nov-07
POSjet
ENQ
Data
Data
Proc.
ENQ Response
Buffer Getting Full
Clear
DTR
Set
Buffer Getting Empty
Select Key

Figure 36 Serial Port Flow Control Using DTR

Rev M
®
1500
Data
Print
Buffer
40 - 8K
DTR = Data Terminal Ready
Port Protocols and
Connection Specifics
Printer Control
Software
Data
Page 259
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