Signal Levels; Serial Port Protocol; Figure 27 Serial Port Flow Control Using Dtr - TransAct Ithaca 90PLUS Series Programmer's Manual

Communications
Serial Port
8.3.2.1
The serial interface meets EIA RS-232 requirements.
Logic 1
Logic 0
8.3.3

Serial Port Protocol

The serial port supports two flow control standards, XON/XOFF and Ready/Busy (sometimes called DTR
or hardware handshake).
When Ready/Busy flow control is selected, the printer can be configured to use DTR, 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 discussions will assume 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
will continue to except up to 255 15 bytes of data after it indicates that it is not ready. Figure 27 illustrates
how the Ready/Busy protocol works.
Communications
Port
Serial
Data In
Data
Inquire
Serial
Responce
Data Out
(Not Used For
Flow Control)
DTR
DTR
RTS
RTS -> Request To Send

Figure 27 Serial port flow control using DTR

15
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.
Page 138
PcOS Series 90PLUS

Signal Levels

-15 V to -3 V: mark = off
+3 V to +15 V: space = on
Serial Port Flow Control
Using DTR
ENQ
High
Speed
Data
Proc.
FIFO
ENQ Responce
Buffer Getting Full
Clear
Set
Buffer Getting Empty
Select Key
DTR -> Data Terminal Ready
Data
Data
Print
Buffer
256 - 6K
Rev Q
Programmer's Guide
Printer Control
Software
11/07/05