Instruction manual

A758 Owner’s Guide Chapter 7: Programming Guide
September 1999 165
In this case the sequence will also be handled correctly as the graphics data it is
intended to be when the graphics command is executed from the buffer.
Third, care must be taken not to insert a Real Time command into the data
sequence of another command that consists of two or more bytes.
In this case the printer will use the real time command sequence bytes instead of
the other command’s parameter bytes when finally executing that other
command from the buffer; the other command will NOT be executed correctly.
These three situations generally preclude use of standard DOS drivers for the
serial communication ports when using real time commands.
Moving Data Through the Buffer
Another consideration is that an application should take care not to let the buffer
fill up with real time commands when the printer is busy at the RS-232C
interface. A busy condition at the RS-232C interface can be determined by bit 3 of
the response to 1D 05 or 1D 04 1 or 10 04 1. The reason for a particular busy
condition can be determined by other responses to 1D 04 n or 10 04 n.
Although the printer responds to Real Time commands when it is busy, it will
place them into the buffer behind any other data there, and flush them out in the
order in which they were received. When the printer is busy due simply to buffer
full (that is, it can’t print data as fast as it can receive it), then data continues to be
processed out of the buffer at approximately print speed and the Real Time
commands will eventually get flushed out.
When the printer is busy due to an error condition, then data stops being
processed out of the buffer until the condition clears one way or another. In
either case, but more quickly in the case of an error condition, the buffer can fill
with real time commands.
When the DLE sequences are being used, the last byte stored when the buffer fills
up could be the DLE code, with no room for the subsequent EOT or ENQ. When
this lone DLE byte is finally processed out of the buffer it will be interpreted as a
Clear Printer command.
Similarly, when the GS sequences are being used, the last byte stored when the
buffer fills up could be the GS code, with no room for the subsequent EOT or
ETX or ENQ. When this lone GS byte is finally processed out of the buffer it will
use the next byte, whatever it is, as the second byte in its GS sequence.
To guard against this situation, an application should determine the cause of a
busy condition and take appropriate action or pace further real time commands
to avoid filling the buffer. There are a minimum of 256 bytes available in the
printer’s buffer when it goes busy.