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Sony SOBAX ICC-2550W Programmable Electronic Calculator

This wonderful machine is a prime example of Sony's second generation of desktop electronic calculators. This second generation of calculators from Sony leveraged integrated circuit technology, versus the hybrid transistorized circuitry of first generation machines, such as the Sony SOBAX ICC-500W. The 2550 offered high-end functionality for the discriminating calculator buyer in the early 1970's. It provides the four functions plus square root, 253 step programmability, nine memory registers, offline program storage on magnetic cards, built-in printer interface, and of course, the extremely high quality that Sony was known for.

A view with the display cover in place

The 2550 is very nicely put together, in classic Sony style. The base of the cabinet is a heavy plastic casting, and the upper half of the cabinet is a combination of plastic mated with a rather beefy aluminum casting that serves as the keyboard/display panel. The upper section of the case is removable by taking out two screws, sliding the upper section of the cabinet toward the rear of the machine slightly, and lifting it off. A removable hinged cover is provided that covers the display panel, providing both protection, and a more aesthetic look for the machine when it is not in use. The back panel of the machine has connections for the power cord, a switched auxilliary power outlet (for providing power to an external device, such as the optional printer), and a port for plugging in an external printer.

Profile view of the Sony Sobax 2550.

The Sobax ICC-2550W is a 15-digit programmable desktop calculator. Fifteen digits is an unusual number for the capacity of a calculator -- most machines use sixteen or fourteen. The reason for this is that the machine actually calculates results to 16 digits, but the 16th digit is a hidden guard digit, used for round-off functions and improved accuracy. The display panel houses the fifteen individual Nixie tube displays, along with 36 discrete incandescent lamps for showing the status of programming functions, sign, and error conditions. The calculator operates conventionally for calculators of the time, with addition and subtraction working arithmetically, and multiplication and division using algebraic entry, with an [=] key to calculate the result for such operations. Decimal point location is fixed, and set manually by the user using two keys that move the decimal point to the left or right. A [CHG SIGN] key toggles the sign of the number currently in the display, allowing the calculator to properly handle mixed-sign operations. The 2550 has 9 memory registers, numbered 1 through 9, that can be stored into using the [M IN] key, recalled from with [M OUT], and cleared using [M CLR]. The memory register to be operated on is specified by a single digit keypress following the memory function key. Memory registers can also be added to or subtracted from by pressing the [M] key, followed by a memory register number, followed by the [+] or [-] key. The [R] key swaps operands on the standard four functions, for example:

2 ÷ 4 R =

would result in 2, because the [R] key would effectively turn the calculation into "4 ÷ 2". The 2550 provides a round up function, activated by the [5/4] push-on/push-off key. The unusual part of this function is that there are a selection of three different digit positions at which the round-off function occurs, selected by a 'one of three' array of switches with symbols on them that match up with designators on the display panel to indicate the digit position where the rounding operation takes place. The round-off operation can be set to occur based on the hidden digit behind the right-most digit on the display, digit 2, or digit 4 (with digit 1 being the right-most digit of the display).

The 2250 with the top part of the cabinet removed

This 2550 was built in early 1970, based on date coding on the early IC's in the machine. All 197 of the IC's that make up most of the logic of the 2550 are made by Sony, and all appear to be small-scale devices, with at most a few gates or a flip-flop or two in each package. The logic supply for the IC's is 4 volts, which makes it difficult to tell what kind of logic is used. It definitely isn't conventional DTL or TTL logic, the voltages are too low, but it seems likely that the logic is similar in design to DTL or TTL, just using slightly lower logic levels. The IC's are all 14-pin devices, with power supply provided on Pin 14 (+4V), and Pin 13 (Ground), a very unusual power supply pinout. All the IC's are dual-inline packages (DIP) in plastic cases, and have Sony 500-series numbers, IE: 501, 503, etc. The calculator also uses a large number of discrete components, mostly diodes and resistors.

The six circuit boards of the Sony Sobax 2550 (shown rear to front)

The electronics of the calculator are interconnected via a printed circuit board backplane that contains some circuitry related to keyboard encoding and signal conditioning. The rest of the backplane board holds edge-card connectors mounted at an angle to accept the six plug-in circuit boards the contain the main logic of the calculator.

Side View of Sony Sobax 2550 Internals

The card cage that holds the plug-in boards is made of heavy cast and machined aluminum side rails, and has stamped sheet-metal covers that shield the top and bottom sides of the cage to prevent EMI (electro-magnetic interference) from radiating out of the machine. The circuit cards themselves are made of a phenolic material, and have circuit traces on both sides, with plated-through holes to provide connections between the sides of the board. One of the boards has high enough density that there simply wasn't enough room for traces to make all of the connections, so this board has a number of hand-added jumper wires on the component side of the board to provide the extra interconnections. Many of the circuit boards also have hand-wired modifications made to them, such as a diode or transistor tacked on to either side of the board. In general, it is much less expensive to hand-add such engineering changes rather than make circuit board design changes. Changes to circuit boards are usually 'saved up' until a new production run is needed, then revisions were made to the circuit board artwork and component layouts, and a new production run made with the engineering changes incorporated.

The Delay Line Module

The rear-most board in the card-cage contains the circuitry for driving the acoustic delay line that serves as the working storage for the machine. This card has comparatively few IC's on it, relying mostly on discrete components to shape, condition, time, and drive the bits going into the delay line, along with providing conditioning, re-timing, and level shifting for the bits coming out. The delay line is mounted to the back-side of the controller circuit card, and is about the same size as the card itself. This is by far the most complex and sophisticated delay line I've come across thus far. Given that the 2550 has nine memory registers and 253 steps of program storage, and also given that the machine has no signs of other on line storage such as magnetic core, the delay line module must provide most all of the operating storage for the machine. The delay line in the 2550 has far more capacity than those in other machines in the museum that use the same technology for working register storage. It is interesting to note that the delay line has a label on it saying "UNREPAIRABLE! REPLACEMENT AT SONY SERVICE CENTER ONLY". With as many bits as have to be crammed into the loop of wire making up the delay line, the adjustments to the timing circuitry to drive it must be very critical, which explains why this device was not serviceable in the field.

A view of the back panel of the Sony ICC-2550W

The 2550 provides a reasonable set of programming functionality. The green keys on the keyboard designate the program functions. The main mode of the calculator is set by two keys that are mechanically linked so that only one can be depressed at any time. These keys are labeled [MANUAL[ and [AUTO]. When the machine is in [MANUAL] mode, it acts like a regular calculator, with no programmer functions. When the mode is in [AUTO], then the calculator activates the programmer functions. Three more keys under a lid to protect them from accidental activation control the programming mode of the machine. Two of these keys are mechanically linked so that only one can be depressed at any time, are [PROGRAM] and [CHECK]. When [PROGRAM] is depressed, and the calcualtor is in "AUTO" mode, keypresses are stored in program memory. This mode is used for entering programs. When [CHECK] is depressed, the steps of the program can be verified step at a time, via indicators on the display panel of the machine that light up for each key that is stored in program memory. This mode can be used to debug programs. Last of the three program mode keys is the [CORR] key, a momentary contact switch that is used to allow program instructions to be edited by replacing a program step with a different instruction. During program entry and debugging, each step of the stored program is displayed via incandescent indicators of the display panel of the machine. An indicator exists for each key on the keyboard, and as each keycode in program memory is encountered, the corresponding indicator lights up. This method of indicating program steps is similar to that of the Burroughs C3660. Other green keys on the keyboard provide the ability to program direct and indirect branches, conditionals, and other programmed functions such as marking the end of a program and control of the optional printer. Lastly, the green [S] key acts as a toggle for starting/stopping execution of the program stored in memory.

The controls for the built-in magnetic card reader are located on the right-hand side of the keyboard. A group of three keys, mechanically linked so that only one can be depressed at any given time, control the mode of the card reader, providing [ENTER], [VERIFY], and [RECORD] functions. The card reader cycles by itself when a card is inserted in the slot at the lower right edge of the keyboard, drawing the card in, then ejecting it back out the way it came. When in "ENTER" mode, the program stored on the card is read into program memory. When in "VERIFY" mode, the content of the card is compared with that in program memory, and if a discrepancy exists, an ERROR condition is generated. Lastly, when the card reader is in "RECORD" mode, the program memory is written out to the card.

Text and images Copyright ©1997-2019, Rick Bensene.