TransAct Ithaca 280 Programmer's Manual page 296

Communications
Flow Chart 2 illustrates how the print control software takes data from the buffer and
controls flow. At the top of the 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.
When XON/XOFF flow control is used, the flow is similar to DTR flow except that DTR is
not used and XON/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 the
buffer.
Move buffer pointers to
the next data byte.
Is the buffer
below the low
watermark?
No
Return data to print
controller.
Flow Chart 2 Print Controller Using Data
Page 284
iTherm
Loop through idle
task, and then look
for more data.
Return to print controller
No
with no data available.
Yes
Set DTR or transmit
XON.
Rev K
®
280 Programmer's Guide