HP 24SII - Manual (Page 65)

If a computer wishes to clear the buffer, the computer sends the Port
Select Code followed by
CLR and a terminating character
(Enter/Return or Line Feed) to the Model 24SII DES.
This command will only affect print jobs originated by that particular
computer. Print jobs in the buffer from other computers are not
affected. The clear buffer command will clear print jobs currently
queued in the buffer and also a print job in process, if one is printing.
NOTE: This command will not clear a printer's buffer of data already
passed to it from the Model 24SII DES.
The clear buffer command is operable in both Port Select Mode A or
Mode B. However, in Port Select Mode B, the Model 24SII DES is
transparent after the first 16 characters are received, therefore the
computer must wait for the Input Inactivity Timeout to occur before
sending the clear buffer command.
The clear buffer command may be sent as a separate ASCII file
preceding the print job, via DOS batch files, as part of a printer
initialization string, or by using the utility software.
*
%
Each print job must be a minimum of three (3) characters. The Input
Inactivity Timeout interval is not valid until after three characters are
received by the Model 24SII DES.
If less than three characters are received within the inactivity timeout
interval (default is 20 seconds) after the first character(s) is received,
the character(s) will be discarded.