Ithaca ITHERM 280, Programmer'S Manual

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Flow Chart 1 illustrates how the communications driver acquires data from the serial port
and places it in the buffer using Ready/Busy or XON/XOFF flow control.
Flow Chart 1 Serial Buffer Operation
At the top of this flow chart, 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 DTR to indicate to the host
that no more data should be sent.
Wait for data from the
host.
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