Editing IBM 1620 (section)

===Implementation "levels"===
* [[IBM 1620 Model I|Model I]]
** Level A; prototype.
*** All [[Flip-flop (electronics)|flip-flop]]s in the design were [[transistor]]ized versions of the original ''Eccles-Jordan trigger circuit''. While this machine was fully functional, it was found that the [[capacitor]] coupling used in these proved troublesome in the noisy signal environment of [[relay]]s and timing [[Cam (mechanism)|cam]] driven [[switch]]es used to drive the console typewriter. This necessitated a ''complete redesign'' of the machine to use ''S-R'' flip-flops instead (except for two triggers used to generate clocks for the ''S-R'' flip-flops). However usage of the term ''Trigger'' was retained in all the documentation when referring to a flip-flop, as it was IBM's conventional term (as ''alphamerics'' was their term for alphanumerics).
*** This is the only level using a one piece vertical control panel, when the design was transferred from Poughkeepsie to San Jose it was redesigned to the two piece angled control panel used on all production models.
** Level B; first production.
*** This is the only level using a burnished aluminum lower control panel, later levels finished this panel with white.
** Level C; introduction of 1622 card reader/punch.
** Level D; introduction of 1311 disk drives and addition of optional "Gate J" containing disk control logic.
** Level E; introduction of Floating-Point option.
** Level F
** Level G; introduction of Interrupt option (needed for [[IBM 1710]]).
*** Did not support BT & BB subroutines in interrupt code!
*** Disk control logic on "Gate J" logic merged into "Gate A" & "Gate B".
**** Made possible because much of logic was compacted using cards designed for the Model II.
** Level H; improved Interrupt option that supported BT & BB subroutines in interrupt code.
*** Final version of the Model I.
* [[IBM 1620 Model II|Model II]] (no information on "Levels" available at this time)
:: The 1620 Model II introduced basic [[arithmetic logical unit|ALU]] hardware for addition and subtraction (making "'''C'''an't '''A'''dd, '''D'''oesn't '''E'''ven '''T'''ry" no longer applicable) and [[index register]]s.
* Model III
** Work was begun on a 1620 Model III but the project was quickly canceled as IBM wanted to promote sales of their new [[System/360]] and discontinue the old lines.