Editing IBM 1620 (section)

==Hardware implementation==
[[File:SMScard.jpg|thumb|A [[Standard Modular System]] (SMS) card]]
Most of the logic circuitry of the 1620 was a type of [[resistor–transistor logic]] (RTL) using [[drift-field transistor|"drift" transistor]]s (a type of transistor invented by [[Herbert Kroemer]] in 1953) for their speed, that IBM referred to as [[Saturated Drift Transistor Resistor Logic]] (SDTRL). 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'' (another type of RTL, but slower than ''SDTRL''), ''CTDL'' (a type of [[diode–transistor logic]] (DTL)), and ''DL'' (another type of RTL, named for the kind of transistor used, "drift" transistors). Typical logic levels of all these circuits (''S Level'') were high: 0&nbsp;V to -0.5&nbsp;V, low: -6&nbsp;V to -12&nbsp;V. [[Transmission line]] logic levels of ''SDTRL'' circuits (''C Level'') were high: 1&nbsp;V, low: -1&nbsp;V. Relay circuits used either of two logic levels (''T Level'') high: 51&nbsp;V to 46&nbsp;V, low: 16&nbsp;V to 0&nbsp;V or (''W Level'') high: 24&nbsp;V, low: 0&nbsp;V.

These circuits were constructed of individual discrete components mounted on single sided paper-epoxy [[printed circuit board|printed circuit]] boards {{convert|2.5|by|4.5|in|mm|abbr=off|sp=us}} with a 16-pin gold-plated [[edge connector]], 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]] [[Integrated circuit#Small-scale integration (SSI)|SSI]] or simpler [[Integrated circuit#Medium-scale integration (MSI)|MSI]] package (e.g., 3 to 5 logic gates or a couple of flip-flops).

These boards were inserted into sockets mounted in door-like racks which IBM referred to as ''gates''. The machine had the following "gates" in its basic configuration:
* "Gate A"{{snd}} Forward hinged gate that swings out the back for access, after "Gate B".
* "Gate B"{{snd}} Rear hinged gate that swings out the back for access.
* "Gate C"{{snd}} Slides out back for access. Console Typewriter interface. Mostly [[relay]] logic.
* "Gate D"{{snd}} Slides out back for access. Standard I/O interface.

There were two different types of [[core memory]] used in the 1620:
* Main memory
** Coincident Current X-Y Line addressing
*** 20,000, 40,000, or 60,000 Digits
** 12-bit, even-odd Digit Pair
** 12 one-bit planes in each module, 1 to 3 modules
*** 10,000 cores per plane
* Memory Address Register Storage (MARS) memory
** Word Line addressing
*** 16 Words, minimum of eight used in basic configuration
*** Single Word read, multiple Word clear/write
** 24-bit, five-digit decimal Memory Address (no 8 - Ten Thousand bit stored)
** 1 plane
*** 384 cores
The address decoding logic of the Main memory also used two planes of 100 [[pulse transformer]] cores per module to generate the X-Y Line half-current pulses.

There were two models of the 1620, each having totally different hardware implementations:
* [[IBM 1620 Model I|IBM 1620 I]]
* [[IBM 1620 Model II|IBM 1620 II]]