Home Page
IBM 1401 Description

The 1401 Data Processing System is a serial, variable word length, stored-program machine. It is controlled entirely by stored-program instructions; there are no control panels in any of the units.

Three units compose the 1401 Data Processing System:

1. The 1401 Processing Unit. In it is found the circuitry necessary to perform machine functionsó arithmetic operations, comparing, forms control, etc.; it also houses core storage and the console.

The console consists of
(1) buttons and switches necessary to start routines, stop routines, test routines in the planning stages, and restart a routine after it is stopped; and

(2) display lights which indicate the contents of storage positions and functions which are occurring.

2. The 1402 Card Read Punch. All card reading and punching occurs in this unit. The reading occurs in the right-hand side at a maximum of 800 cards per minute. Punching occurs in the left-hand side at a maximum of 250 cards per minute.

Cards to be read pass two sets of reading brushes. The first set of reading brushes is used for a checking function; the second set completes the check and allows the data to enter storage.

On the punch side, cards first pass a set of punches where punching occurs and then a checking station where the punching is checked.

Both feeds have a "pretest" station for sensing misfeeds. All card input and output is checked for validity. Invalid codes cause the machine to stop.

3. The 1403 Printer prints at a maximum of 600 lines per minute and has a print span of 100 positions or 132 positions. Horizontal spacing is 10 characters per inch. Any one of 48 charactersó26 alphabetic, 10 numerical and 12 special ócan print in each print position. The twelfth special character is the + symbol or the record mark, depending upon the 1401 model.

Output to the printer is checked.

Vertical spacing can be six or eight lines to the inch and is under operator control.

Up to six magnetic tape units can be attached to the 1401 Data Processing System. They can be either the 729 II or 729 IV.

1401 Storage

There is a maximum of 4,000 positions of core storage. Each position has a three-digit address.

Three areas of storage are reserved for specific purposes:
(1) positions 001-080 for card read-in,
(2) positions 101-180 for punching, and
(3) positions 201-300 (or 201-332) for printing. When not used for these specified purposes, these areas may be used for any other purpose.

In the 1401, characters are in the binary coded decimal; a parity check is performed for each character on every move.

Instruction Format and Operation Codes

The 1401 processes an application by executing a series of instructions. These instructions must first be determined by the programmer, written, punched into program load cards, tested and then stored in the machine. The machine must have all necessary instructions stored in it before processing the data.

Personal Reflections - Roger Mills:

At a Digital Computer Association (DCA) meeting, the Western Regional Representative for IBM was going to give us a talk on the IBM 1401.  It so happened that a 1401 design engineer from IBM was out here and the rep decided that he could present the talk on the IBM 1401.  My guess is that this guy had never been west of the Mississippi and had no idea the types of character that attended the DCA meetings.  To give you a clue, DCA was also known as the Drunken Computer Association. After the talk the presenter open things up for questions.  That was his second mistake. The first question was "How did the character chain operate?  Did it stop each time to print or what?"  Unfortunately, the presenter didn't know the answer and tried to snow the audience.  Bad Idea!  The audience really went after him and in the middle of this, Jack Strong walked in from the bar and asked, How many 407s will a 1401 replace?"  The presenter figured this questioner was a drunk who wandered in from the bar and gave him some ridiculous answer. Jack replied, "I'm a little interested since I have just order 14 of them."  At that, one of the audience shouted, "Jack, you order 14 and you don't know what they will do?'  Another piped up, "But he will be the firstest with the mostest!"  By this time, the presenter was a white as a sheet and realized he had insulted one of the largest customers of IBM.  My guess is that this guy never came west of the Mississippi again.

The 1401 was to replace the card-to-tape reader unit and the tape-to-card punch.  Since we punched almost entirely binary cards (lots of holes per card), the 1401 punch unit failed in little or no time.  The designers assumed that only decimal cards would be punched and small amount of binary cards.  IBM went back to the drawing boards and beefed up the punch mechanism.