Figure 30 Print Controller Using Data - TransAct Ithaca 90PLUS Series Programmer's Manual

Programmer's Guide
Figure 30 illustrates how the print control software takes data from the buffer and controls flow. At the top
of the flow chart, the print control software asks for data. If there is no data in the buffer a "no data flag" is
returned. The print software must then wait for data. If there is data in the buffer, it is read, and the pointers
are updated. The buffer is then checked to see how much information is left. If the buffer is below a low
watermark (about 100 bytes left), the communication driver is notified, and DTR is reasserted.
If XON/XOFF flow control is being used, the flow is similar to DTR flow except that DTR is not used and
XON and XOFF control characters are transmitted back to the host on the serial link. The XON/XOFF
advantage is that only three wires are required to interconnect to a printer. The disadvantage is that a serial
port receiver driver must be written for the host.
The print controller
needs data to print or a
command to decode.
Is there any
data in the
buffer?
Yes
Acquire data from buffer.
Move buffer pointers to
the next data byte.
Is the buffer
below the low
watermark?
No
Return data to print
controller.

Figure 30 Print controller using data

The print controller is always looking for data when the printer is on-line and not printing. It continuously
asks the buffer if it has any data. If there is no data in the buffer, it does housekeeping chores like keyboard
scanning. If there is data, it processes it. The flow control is done when the data is taken from the buffer and
the amount of data in the buffer is less than a prescribed amount. The setting for the low watermark is based
on the expected environment. The Series 90PLUS Printer sets this at 200 characters, which gives the host
application time to get more data to the printer before the printer uses up what it has.
11/07/05
PcOS Series 90PLUS
Loop through idle
task, and then look
for more data.
No
Return to print controller
with no data available
Yes
Set DTR or transmit
XON.
Rev Q
Communications
Serial Port
.
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