Ithaca PcOS series 90plus, Руководство программиста

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Programmer's Guide PcOS Series 90PLUS Communications
Serial Port
12/16/99 Rev M Page 147
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.
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.
Wait for data from the
host in high speed first-in
first-out (FIFO) buffer.
Place data in the print
buffer.
Is the
buffer full?
Clear the DTR signal or
transmit XOFF.
Update the buffer
pointers.
Is the buffer
past the high
watermark?
No
Yes
No
Yes