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Programmer's Guide PcOS Series 150 Communications
Serial Port
12/14/99 Rev G Page 121
The following flow chart illustrates how the print controller takes information from the buffer
and controls flow. At the top of the chart, the print control software asks for data. If there is no
data in the buffer, a no data flag is returned. The print software must then wait for data. If there
is data in the buffer, it is read and pointers are updated. The buffer is then checked to see how
much information is left. If the buffer is below a low watermark (about 100 bytes left), the
communication driver is notified, and DTR is reasserted.
If XON/XOFF flow control is being used, the flow is similar to DTR flow except that DTR is not
used and XON/XOFF control characters are transmitted back to the host on the serial link.
XON/XOFF has an advantage over DTR in that only three wires are required to interconnect to a
printer. The disadvantage is that a serial port receiver driver must be written for the host.