I don't think that anything like PROMPT or INPUT is built-in with
the 28 series, but as Marcus noted, the HALT command seems to be
the obvious choice for halting a program to allow user input.
Bill Wickes lists a PROMPT program in HP-28
Insights.
<< "Enter " SWAP +
CLLCD 1 DISP
HALT
>>
Of course, with the above program, the prompt string will always
start with "Enter ".
If you want a program where you specify the complete prompt
string, like the built-in PROMPT command in later RPL models, then
simplify it to:
<<
CLLCD 1 DISP
HALT
>>
You could store either of the above in a global variable to be
called by name in other programs, much like a built-in command, or
you could include:
"prompt string" CLLCD 1 DISP HALT
as in-line code in other programs.
Regards,
James