IBM 1401 - Hardware Implementation

Hardware Implementation

Most of the logic circuitry of the 1401 was a type of diode-transistor logic (DTL), that IBM referred to as CTDL. Other IBM circuit types used were referred to as: Alloy (some logic, but mostly various non-logic functions, named for the kind of transistors used), CTRL (a type of resistor-transistor logic (RTL)). Later upgrades (e.g., the TAU-9 tape interface) used a faster type of DTL using "drift" transistors (a type of transistor invented by Herbert Kroemer in 1953) for their speed, that IBM referred to as SDTDL. Typical logic levels of these circuits were (S & U Level) high: 0 V to -0.5V, low: -6 V to -12 V; (T Level) high: 6 V to 1 V, low: -5.5 V to -6 V.

These circuits were constructed of discrete components (resistors, capacitors, transistors) mounted on single sided paper-epoxy printed circuit boards either 2.5 by 4.5 inches (63 by 110 mm) with a 16-pin gold plated edge connector (single wide) or 5.375 by 4.5 inches (136.5 by 110 mm) with two 16-pin gold plated edge connectors (double wide), that IBM referred to as SMS cards (Standard Modular System). The amount of logic on one card was similar to that in one 7400 series SSI or simpler MSI package (e.g., 3 to 5 logic gates or a couple of flip-flops on a single wide card up to about 20 logic gates or 4 flip-flops on a double wide card).

The SMS cards were inserted in sockets on hinged swing out racks, that IBM referred to as gates.

Like most machines of the day, the 1401 used magnetic-core memory. The cores were about 1 mm in diameter and used a four-wire arrangement (x, y, sense, and inhibit). The memory was arranged in planes of 4000 cores each, each core storing one bit. A stack of eight such planes stored the six data bits, word mark bit, and parity bit for 4000 characters of storage. Together with eight additional planes with fewer cores on them for additional storage functions, this made up a 4K memory module. One such module could be housed within the 1401's primary enclosure. Systems were commonly available with 8K, 12K, or 16K of memory, the additional modules being contained in an add-on box, the 1406 Core Memory Unit, which was about two feet square and three high. Operands in memory were accessed serially, one character at a time, and the 1401 could read or write a character within its basic cycle time of 11.5 microseconds. All instruction timings were cited in multiples of this cycle time.