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Logic Schematic for the "Three-Counter" version of the Friden EC-130 Calculator
Late 1965
This is a logic-level block diagram for the Three-Counter version of Friden's first electronic calculator, the Friden EC-130. The Three-Counter version of the EC-130 replaced the earlier Four-Counter implementation when it was found that the "B" counter was redundant, and that the circuitry could be simplified by eliminating the B counter, leaving the A, C, and D counters to provide the counting functionality needed. This change required a bit more control logic, but it was more than made up for in component count reduction from the elimination of the B counter, with its five flip flops and associated gating circuitry. The Three-Counter architecture was implemented in EC-130 calculators beginning with serial #8500.
Note that this schematic includes the logic for the optional (and very rare) Entry Counter. This option provided an electromechanical counter that would be installed such that its content would show through a cutout in the CRT panel of the calculator, to the right of the CRT display. The counter can be used to keep track of the count of entries made into the machine, useful for calculating averages or item counts for invoicing purposes. The counter had a manually activated button on it to set it to zero.
There is no date listed on this document, but it is estimated that this revised logic schematic was made sometime in the late part of 1965 as other documentation relating to the Three-Counter change is from this same time period.
The logic block diagram includes all of the logic elements of the calculator at the gate and flip-flop level. The schematic includes only the logic circuitry located on the plug-in circuit boards, and does not include the power supply, details of the write driver and read amplifier for the delay line, as well as details of aspects of the CRT drive and deflection circuitry. Many of the details of this "non-logic" circuitry can be found in the Service Manual.
Given that the EC-130's logic is implemented entirely with discrete components the schematic uses unusual symbology and nomenclature to designate the logic elements. The general logic function is the OR gate, which is shown in the schematic as a box with an "O" at the top center. Boxes with an "I" in the top center are inverters. Boxes with an "A" designation are AND gates, and boxes with "EF" are transistor emitter follower circuits, used on the output of gates to allow higher fanout capacity for gates that need to drive many other gate inputs. Boxes that are divided in half by a horizontal line represent flip flops. Some flip flops have "EF" designations on their outputs, indicating that the outputs use an emitter follower to provide higher fanout for driving other logic. Other small boxes with diamond-shaped symbols on inputs are called AC gates, which operate on logic level transitions rather high or low states. The logic design is well described in the EC-130 Service Manual, with some additional information in the EC-130 Field Service Manual. The EC-130 Service Manual Supplement, which replaces Section 5 of the original (Four-Counter) EC-130 Service Manual was published in December of 1965, outlining the differences in the Three-Counter EC-130 as compared to the Four-Counter version.
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