Alternate Implementation; Rules For Using Real Time Commands - NCR 7158 Owner's Manual

Chapter 7: Programming Guide 7-101

Alternate Implementation

The alternate implementation uses the DLE (0x10) sequences as
implemented on other printers. An application using these DLE (0x10)
sequences and the original 7156 Clear Printer command (0x10) must
distinguish for the printer between the new real time commands and
the Clear Printer command by adding a NUL (0x00) to the Clear
Printer command.
An application using these DLE (0x10) sequences must also send the
second byte of the sequence within 100 milliseconds of the first, to
prevent the first byte being mistaken for a Clear Printer command.

Rules for Using Real Time Commands

Three situations must be understood when using real time commands.
First, the printer executes the Real Time command upon receiving it
and will transmit status regardless of the condition of the DSR signal.
Second, the printer transmits status whenever it recognizes a Real Time
Status Transmission command sequence, even if that sequence
happens to occur naturally within the data of another command, such
as graphics data.
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.