The card reader/punch, the printer, and the I/O cores
One unusual feature of the core module is the eight special-purpose I/O frames: six core planes and two terminal frames. To understand the I/O cores, some background on the IBM 1401 is necessary. The 1401 was used in business applications such as accounting and payroll, so accuracy was extremely important. If a malfunction caused bad payroll checks to be printed, it would be a catastrophe. To catch problems, IBM put many types of validity checking into the 1401, making it much more reliable than competitors. The basic I/O devices for the 1401 were the card reader/punch and the line printer, separate units from the computer itself and the I/O cores detected problems with these devices.The I/O planes are addressed exactly the same as the data planes. However, the I/O planes are very sparse, with only 297 cores rather than 4000 cores, so most locations have no storage as can be seen in the photo below. These planes are accessed by the I/O circuitry, and are invisible to the programmer.
Closeup of the IBM 1401’s core memory. The row bit core planes are used for I/O and are sparsely populated.
The IBM 1401 uses 80-character punch cards. You might expect the card reader to read each character on the card in sequence and send the character to the computer, but that’s not at all how it works. Instead, the card reader processes each card “sideways” for speed, using 80 metal brushes to read a row at a time. If a card has a hole in a position, the brush contacts a metal roller under the card, completing a circuit. The brushes are connected to the IBM 1401 by 80 wires, one for each brush. Each wire is connected directly to a “row-bit core” in the core memory module, setting the core if a hole was detected. There’s no driver circuitry or memory addressing; it’s literally a separate wire from each brush that is wrapped 5 times around a core. Let me emphasize how unusual this is: it’s like having a separate wire from each key on your keyboard directly to a specific transistor in your memory chip.
The card reader/punch has three read stations: RD1 and RD2 for reading, and PCH for reading after punching. Since each read station has 80 brushes, 240 wires connect the brushes to the 240 row-bit cores. (As you might have guessed, the cables between the 1401 computer and the reader/punch are very thick.) As well as the row-bit cores, reading/punching uses core planes called XU, YU, XL, and YL to count the number of holes detected in each position. If the two read stations have different hole counts, the computer stops and reports a fault. Likewise, the count is checked after punching a card to make sure all holes were punched correctly.
The high-speed line printer uses 132 hammers to produce 132-column output. A chain with the 48 printable characters whizzes around horizontally. As each character on the chain passes a position where it should be printed, a hammer fires at the precise time, hitting the paper against the inked ribbon to print the character. The I/O cores are also used to detect problems in the printing process.
Printing uses several different core planes for multiple validity checks. Each of the 132 print hammers is wired directly to a “hammer-fire core” in the memory module. The XU core plane is used during printing for the print-compare check: a bit is set in the XU plane if a hammer should fire for the character position. These 132 bits are compared with the hammer-fire cores to verify that the correct hammers fired. Plane YL holds print-line complete cores that verify that every character position either printed a character or holds a non-printable character. Finally, to aid printer maintenance, plane YU records the location of any fault in print-error storage core.
