An HP9100B core memory moduleMemory for a programmable calculator
Part of
the Core Memory pages
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An HP9100B core memory moduleMemory for a programmable calculator
Part of
the Core Memory pages
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Various sources document this calculator as having 32 registers, each able to hold either 12 decimal digits (2 exponent, 10 mantissa) or 12 6-bit instructions, plus a 3-level stack (the X, Y and Z registers). This is 35 registers of 72 bits each, or a total of 2520 bits. This is considerably less than the 4416 bits reported by some sources, but this is a deeply microprogrammed machine with primitive operations in its user-level instruction set that include trig functions. Presumably, much or all of the rest of the core memory is used for variables needed by the microcode to implement the calculator's user level instruction set.
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The use of mother board wiring for communication between the two boards of the module suggests that the mother board is not organized with any kind of bus structure. Rather, it appears that the different slots are each dedicated to a specific function, and that the mother board wiring is random, supporting those functions. In fact, most of the area of the mother board is devoted to huge numbers of diodes that make up the ROM holding the low-level microcode.
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The central feature of this board is the core plane itself. The speculation that the top connector is used for communication with the addressing logic is confirmed by the fact that the printed circuit traces from this connection all lead directly to the core plane, along with 20 of the traces from the front mother board connector.
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Becaise the pins joining the daughter board to the sense/inhibit board are soldered at both ends, removing the daughter board for direct examination of the core plane would pose a great risk of permanent damage. Therefore, the best we can do is peer into the congested space between the boards and look at the shadows visible through the boards. A bit of image enhancement was used to bring out the details of the circuit traces on the hidden side of the daughter board.
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On the Y axis, there appear to be 16 connections at the top left and 16 connections at the top right. These are not aligned with each other, so there are not 16 lines that go across the core plane there. Rather, it would seem that the 16 connections from each side cross the top patch of core and then weave down across a middle patch before exiting at the bottom where there is a row of 16 connections visible in shadow on each side, connected to a common return trace.
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Close examination of the group of 10 connections on the middle of each side of the core plane shows that each group is arranged as 3 groups of 3, with traces from each group diverging to one of the 6 sense-amplifier inhibit-driver clusters. the 10th connection on each side used for a common pin connected to many wires. If there are 3 sense-inhibit connections to each of 6 patches of core, this implies that 4 wires pass through each core, X, Y, sense and inhibit. It is likely that each end of the sense line is connected to two pre-amplifier transistors in each cluster, while the inhibit line goes from a power driver transistor to a common return point.
In any case, the actual core memory plane appears to occupy an area 1.625 inches (41.3 mm) wide by 3.25 inches (82.6 mm) high.
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There is considerable documentation available for the HP9100A. HP published
detailed descriptions of the machine in the
Hewlett-Packard Journal, Sept. 1968
and many of the available peripherals are documented in
Hewlett-Packard Journal, Oct. 1970.
Unfortunately, neither of these publications covers the detailed organization
of the HP9100B core memory.
Pages 82-85 of Hewlett-Packard's
1970 Electronics for Measurement, Analysis, Computation
catalog gives the price of HP9100B as $4900 and states that the machine has
3840 bits of core memory.
Page 61 of
Service Manual, Hewlett-Packard Calculators Models: 9100A, 9100B
lists the Core Driver Ass'y/ Core Sense Amplifier as a single field
replacable part (part number 9100-69595), to be returned to HP for repair if
replaced.
Most end-user discussion of the speed of the 9100 focuses on the time per
floating point add, typically 2 milliseconds. Because the machine operates
on floating point numbers in a digit-serial manner, this involved lots of
memory references. Digging deeper, it appears that the core memory had a cycle
time of 1.6 microseconds. This is given in fine print in the Specifications
on page 9 of the
Hewlett-Packard Journal, Sept. 1968.
The calculator was purchased new from HP by Dr. Charles Wunder, and came to
this collection in operating condition when he retired from his position
in the Physiology department. Wunder and James Van Allen were the two faculty
members at Iowa who made the most extensive use of calculators in the HP9100
family. Wunder's calculator was equipped with
a digitizing tablet, plotter and printer.
Searching for evidence of the actual data of manufacture turns up the following
markings:
None of these appear to be dates, unless the leading digit on the quality
assurance stamps is the last digit of the year, indicating that the board
was made in 1969. If so, the 132 and 138 might indicate the day in the year,
in which case, 9-132 would be Monday May 12, 1969, and 9-138 would be
Sunday, May 18. (According to Rick Bensene, HP's 9100 production line
was running 7 days a week, so these dates are plausible.)
Some of the electrolytic capacitors on the core sense-inhibit board
by the side of the daughter board are marked:
It is likely that the 7013 on these capacitors is a manufacturing date
code, in which case, these capacitors were made in the 13th week of 1970
(in April) and the board must have been assembled later, probably later
that year.
In this case, the QA date stamps on the board are the quality control dates
for the board etching and plating, not the final assembly date, and HP would
have made a batch of board over a year in advance of their final assembly.
The feed rollers for the printer and the magnetic card reader used for
program storage have long-since turned to goo, but the calculator itself was
fully functional when last powered up.
Provenance
Scratched at the top of the board: HPS07AEP (or HP507AEP)
a serial number?
A quality assurance stamp at the lower right: QA 9-132
Scratched at the top right (rear): 122-27
A quality assurance stamp at the top center: QA 9-138
Stamped on the upper right: The letter R
15DC+
1500±10%
22uf
Sprague
7013+
Condition