Punching A Drum Card For A Keypunch Machine
| Field | Computer Programming / Data Entry |
| Went Obsolete | Mid-80s |
| Made Obsolete By | The obsolescence of punch cards |
| Knowledge Assumed | How a keypunch works |
| When useful | Anywhere punch cards still roam in the wild |
Back when dinosaurs ruled the earth and you could buy gas for under a dollar, the normal way of communicating with a mainframe like an IBM 370 was by using punch cards. If you were entering programs or JCL, you usually just punched them freehand, but if you were entering a lot of data cards with a common format, you'd punch a drum card (also called a Program card).
The problem, you see, is that the IBM key punch machines, like the ASR 33 teletype, didn't have a separate set of keys for numbers. Instead, they were overlaid over the letter keys, much the same way that the numeric key pad is overlaid on many laptops. If you wanted to type a number, you hit a magic key, and then the letter that doubled as the number. If you were punching a lot of cards with numeric data (phone numbers, payroll, scientific data, etc.), remembering to hit the function key could be tricky, especially if you were a key punch operator and wanted to be fast.
And here is where the drum card came in. On the more advanced key punch machines, there was a drum that sat in a recess above where the currently punching card was held. You could wrap a punch card around it. By punching different codes into each of the 80 columns on this card, you told the keypunch what to do when it reached the corresponding column on the card you were punching.
It was a marvelous time saver. You could tell it to duplicate whatever was on the last card (for card formats where you put a certain set code in a column), to skip the column altogether, or (and here's the real prize) shift into numeric mode for this column.
Perhaps the most useful function was the automatic numbering code. It would punch a sequence in the columns 72 to 80, which could then be used to put the card deck back in order when it was dropped.
Using a drum card, you could blaze through data entry without ever typing anything but data.
There was also a skill in replacing the “star wheels” - the star-shaped contacts that read through the holes in the drum card. If you forgot to raise the star wheels when you removed the drum card, they would be pulled out of their sockets. It was an additional skill to replace these in their sockets so they could contact the drum through the card.
