Figure 23 Print Controller Using Data - Ithaca 150 Programmer's Manual

Communications
Serial Port
The print controller
needs data to print or a
command to decode.
Is there any
data in the
buffer?
Aquire 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.

Figure 23 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. This low watermark is set based on the expected environment. The Series 150
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.
Page 122
PcOS Series 150
Loop through idle
task, and then look
for more data.
Return to print
controller with no data
No
available.
Set DTR or
Yes
transmit XON.
DTR = Data Terminal Ready
Rev G
Programmer's Guide
12/14/99