TransAct POSjet 1500 Programmer's Manual page 276

Port Protocols and
Connection Specifics
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.
When the printer is on, the print controller looks for data. If there is data, it processes it. 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 low
watermark is set based on the expected environment. The POSjet
buffer size or 1024 characters whichever is smaller. The low watermark gives the host application time to get more
data to the printer before the printer uses up what it has.
When XON/XOFF protocol is implemented, it is possible for the host to miss an XON or an XOFF. To prevent this
from causing a communication lockup, the printer sends an XOFF for every character received after the high
watermark is reached. If the printer detects that the serial data link is inactive, it sends out an XON about every two
seconds. When the printer cover is opened, an XOFF is sent. An XOFF is sent even when the internal data buffer is
past the high watermark. This is done to allow the host to know that the printer is not ready.
Page 262
®
POSjet 1500
Loop through idle
task, and then look
for more data.
Return to print controller
No
with no data available.
Yes
Set DTR or transmit
Flow Chart 2 Print Controller Using Data
Rev M
Programmer' s Guide
XON.
®
1500 printer sets the low watermark at half the
Nov-07