A Link to a book that might be of interest. Written at a time when the HP-28s was brand new.
http://books.google.de/books?id=ntGpV3rMjnoC&pg=PA111&lpg=PA111&dq=hp-28s+program+-cartridge+-druckpatrone&source=bl&ots=bGoy0HNCVD&sig=x3W7UOQSJbYuSJRw5pio9if7hgI&hl=sv&ei=1kGuTOqOG4rHswaGh63cDQ&sa=X&oi=book_result&ct=result&resnum=7&ved=0CDYQ6AEwBjg8#v=onepage&q=hp-28s%20program%20-cartridge%20-druckpatrone&f=false
Sorry, the html Link tool did not work on that link. Cut&Paste :-)