Digital Test
Hewlett-Packard 520 Series

HP 521C
HP 524B
HP 524C

Models of the HP 520 Counter Series
Model Digits Display Max. Rate
521A 4 1-of-10 neon bulbs120 / 220
521C 5 1-of-10 neon bulbs120 / 220
521D 4 NIXIE 120 / 220
521E 5 NIXIE 120 / 220
521G 5 1-of-10 neon bulbs1200
522A 5 1-of-10 neon bulbs100
522B 5 1-of-10 neon bulbs100
523C 6 NIXIE 1200
523D 6 1-of-10 neon bulbs1200
524A 8 6 digits 1-of-10, 2 digits meter 10,000
524B 8 6 digits 1-of-10, 2 digits meter 10,000
524C 8 NIXIE 10,000
524D 8 1-of-10 neon bulbs10,000
ref: HP manuals for 524 and for AC-4 counter modules
During the 1950's Hewlett-Packard manufactured the 520 series of counter instruments. The models of this series span a range of capabilities, varying in the number of digits and maximum count rate.

Appropriate to the period, these instruments are based on vacuum tube technology, with some use of crystal diodes for triggerring flip-flops and level clamping.

Most of the models of the series utilise the HP AC-4 series of decade counter modules. It may be that the earlier models, such as the 524A, were developed before the AC-4 modules.

The 520 series actually has origins in the HP 520A Nuclear Scaler. This was a 10 MHz 2-decade scaler which would feed into a more typical lower speed counter. The 520A circuitry would be integrated with 6 decades of AC-4 counter modules, a timebase and gate control circuitry to become the 524A.

For the lower frequency models (the 521's and 522B), the base model would count to 120KHz. An option was available to increase this to 220KHz, essentially by replacing the AC-4 counter module in the first decade with the appropriate faster AC-4 module. (Note: the source of data for the table of models did not indicate how the 522B differs from the 521C).

While Hewlett-Packard's first calculators and first computers are sought after by collectors, these instruments were HP's first foray into digital technology.

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 The 524

HP 524 Versions
Model Year Front Panel Plug-in Display
524A 1952 No 6 neon columnar, 2 meter
524B 1955 Yes 6 neon columnar, 2 meter
524C 1959 Yes 8 NIXIE
524D 1959 Yes 8 neon columnar
The 524 is the top-of-the-line model of the 520 series. The primary distinguishing feature of the 524 was the ability to count to 10 MHz, how this was accomplished is discussed further below.

Four versions of the 524 were produced: the A, B, C and D. The 524B superseded the 524A by adding provision for various optional input units to plug into the front panel.

The 524 A and B had a somewhat awkward display in which the 6 most significant digits are neon 1-of-10 columnar displays, while the 2 least significant digits are analog meters marked with 10 positions. Each meter is driven by a staircase signal from a simple resistive D-to-A converter in the according digit's decade counter. The 524 C and D superseded the B by eliminating the analog meters to give a consistent display across all 8 digits. The C and D versions were contemporaries, the D continued the use of columnar displays, while the C went to NIXIE tubes for the display. These versions could also drive a special printer, with the aid of an optional kit.

The 524 versions utilise around 90 vacuum tubes, varying slightly depending on the version. The physical form factor changed quite a bit from the A to the B to accommodate the front panel plug-in. The B, C and D are all quite similar physically: a cube of approximately 19-inch rack width (~4 cubic feet), around 110 to 120 pounds (50 to 55 kg) in weight, and consuming 600 watts of power. This is what was required in the 1950's to count to just 10 MHz!

Plug-in Input Units for the 524

Plug-in Input Units for the HP 524 B, C and D
Model Function
525A 10 to 100MHz Heterodyne Converter
525B 100 to 220MHz Heterodyne Converter
525C 100 to 510MHz Heterodyne Converter
526A Video Amplifier
526B Time Interval (separate start/stop inputs)
526C Period Multiplier
526D Phase Angle Measurement
The optional plug-in input units (see table at right) for the 524B, C and D provide for some flexibility in functionality and extension of the maximum measurable frequency.

The heterodyne converters do not actually extend the maximum count rate of the counter. Rather, they split the input frequency such that one portion is a known multiple of 10MHz and the remainder is less than 10Mhz. This is accomplished by mixing a selected harmonic multiple of the crystal-controlled 10MHz timebase of the 524 with the input signal to produce a difference signal of less than 10MHz. The appropriate harmonic is tuned-in by the operator. The frequency of the input signal then is computed (manually) as the sum of the harmonic as indicated on the controls (10, 20, 30MHz ...) and the count indicated in the display (0 to 10MHz).

The plug-in input units show up in HP catalogs as late as 1968, but are not present in the 1969 catalog.

Counting to 10 MHz

Getting the count rate up to 10 MHz was not trivial to accomplish in the tube days, and the 524 presents an example of the limits of using vacuum tubes in the digital mode. The AC-4 decade counter modules are only capable of a maximum count rate of 1.2 MHz. They were thus adequate for the upper 7 digits (MSDs), but not for the units digit (LSD). To get to 10 MHz an exceedingly complex counter for the first decade was required, involving 13 tubes, compared to the 4 tubes for each of the other 7 decades.

The first issue to be addressed in increasing the count rate is the speed of an individual flip-flop. The 524 used 3 different flip-flop designs for different speeds, as shown in the following diagrams:

The low speed flip-flop of the AC-4K module on the left is quite straightforward - note that it uses plate triggering - to count at rates up to 120 KHz.

To improve on this, the flip-flop of the AC-4J moves to grid triggering with the use of diodes, and lower resistances to shorten RC delays. The latter change comes at the expense of increased power consumption of course. A 6211 tube replaces the 5963. These are both 'computer tubes' but the motivation for the change is not clear as the 5963 actually has a higher power rating and lower inter-electrode capacitances. These changes to the counter get the count rate up to 1.2 MHz.

Now look at the flip-flop for the 10 MHz stage:

A 4-bit decade counter involves feed-back resulting in additional timing issues when increasing the speed. While the AC-4 modules got by with simply substituting the higher speed flip-flops into the same decade counter configuration, the 10 MHz stage, once again, involves more significant distinctions. The following diagrams show the logic equivalents of these decade counters for comparison:

Similar complexities for the higher speed show up around the start-stop gate control circuitry of the 524, as the gate must be precise to within a cycle of the maximum count rate.

 AC-4 Series Decade Counter Modules

AC-4 Decade Counter Modules
Model Display Max. Rate
AC-4A 1-of-10 neon bulbs120 [1] 8-pin plug
AC-4B 1-of-10 neon bulbs220 [1] 8-pin plug
AC-4C 1-of-10 neon bulbs220 12-pin plug
AC-4E 1-of-10 neon bulbs1200 12-pin plug
AC-4G 1-of-10 neon bulbs120 12-pin plug
AC-4J NIXIE 1200 two 12-pin plugs
AC-4K NIXIE 120 two 12-pin plugs
AC-4L NIXIE 220 two 12-pin plugs
[1] from 521A/C manual.
These plug-in modules each contain 4 duo-triode tubes formed into 4 flip-flops and interconnected as a decade counter using 1224 bit weighting.

The display is either 10 neon bulbs in a column or a NIXIE tube. In both cases the decoding from 4-bit-binary to 1-of-10 is accomplished by placing 10 neon bulbs in the middle of a bi-quinary (2-by-5) matrix. The 3-bit to 1-of-5 decoding is accomplished with resistors. For the 1-of-10 columnar display the 10 neons are then the display.

For the NIXIE display, the common anode of a NIXIE tube meant it was not possible to simply substitute the NIXIE tube for the neon bulbs in the matrix (bi-quinary NIXIEs would come later). Consequently the NIXIE tube display is accomplished by illuminating 10 CdS photoconductors in 1-to-1 correspondance with the 10 neon bulbs, the photoconductors then controlling the 10 cathodes of the NIXIE. This use of photoconductors would be leveraged in a more complex manner by HP in their 1960s-era discrete-transistor decade counter modules.

Low-speed models of the module count to 120 KHz, medium speed models to 220 KHz, and high-speed models - benefitting from reduced resistor values (shorter RC time constants) and diode triggerring - count to 1.2 MHz.

Various models were superseded over the years, e.g. the 4A by the 4G. Differences seem to be the plug type and some capacitor values. The D, F, and H models may have existed but I have no data about them.

Northeastern Engineering Inc. also manufactured some modules almost identical to the AC-4 series. Physically, there are some minor variations in dimensions and the NE models have side panels covering the circuit board. Electrically, the circuit for NE models 140-40A and 140-100C is identical to that of the HP AC-4A, but with slightly different component values, and the NE models do not include the staircase resistors.

The photos below show three AC-4 models - an AC-4A from 1956, an AC-4G from 1960, AC-4J with NIXIE tube from 1961 - along with a Northeastern 140-100C.



 References, Externals Links

  HP 520 Counter Series
Early Digital Test Equipment
Jul 2005