Print Buffer Flow; Figure 29 Serial Buffer Operation - TransAct Ithaca 90PLUS Series Programmer's Manual

Communications
Serial Port
8.3.4

Print Buffer Flow

The following flow chart illustrates the communications driver getting data from the serial port and placing
the data in the buffer using Ready/Busy or XON/XOFF flow control.
Wait for data from the
host in high speed first-in
first-out (FIFO) buffer.
Place data in the print
Update the buffer

Figure 29 Serial buffer operation

At the top of the flow diagram, the driver is waiting for data. When data is received from the host, the
printer checks to see if it is an inquire command. If the data is an inquire, it is responded to and placed in
the buffer. If not, the data is placed in the buffer without response. The buffer is then checked to see if it is
full before the internal pointer is incremented. If it is not full, the pointer is incremented in preparation for
the next data byte. The buffer is checked by looking to see if it has passed a high watermark. If the buffer
has, the communication driver is notified, and it resets data terminal ready (DTR) to indicate to the host that
no more data should be sent.
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PcOS Series 90PLUS
buffer.
Is the
buffer full?
No
pointers.
Yes
Is the buffer
past the high
watermark?
No
Yes
Clear the DTR signal or
transmit XOFF.
Rev Q
Programmer's Guide
11/07/05