Wang 300 Calc

Rick Bensene rickb at
Thu Sep 6 09:13:17 CDT 2007

The keyboard/display units were simple -- WAY more simple than serial
RS-232.  Logic levels were not TTL (remember, these machines were made
with IC's anywhere either the keyboard/display
units, nor the electronics package).  RS-232 serial did exist back then,
but IC UARTs didn't exist, and everything had to be implemented using
transistor logic, which could fill a small drawer full of circuit boards

If you wanted to interface with one of the 300-Series keyboard/display
units, you'd have to design some level shifters to make standard TTL (or
whatever) levels.    The interface to the display was a 4 bit digit
select (selects which nixie tube is to light up), and a 4-bit BCD code
for the number to light up in that nixie tube.  Since the display was
multiplexed, you just incremented the digit select, and sequentially
presented the digits to be displayed on each tube.  The timing was
somewhat important, as Nixie tubes have some on/off duty-cycle timing
issues, and the cycle rate has to be fast enough that the display looks
continuous, but not too fast, or the digits won't be at full brightness.

The keyboard was encoded by a diode array to a 6-bit code, and a signal
indicating when a key has been pressed.
There were also a few toggle switches on the keyboard that selected
various operational modes which were just unconditioned switch closures.
The keyboard switches were microswitches, which switch pretty cleanly,
but still a little conditioning is needed to get reliable key entry
(e.g., a bit of delay after the "key pressed" signal goes active before
sampling the keycode).

The keyboard/display unit has a cable that comes out of it that
terminates in a telco-style (similar to old Centronics printer
connectors, but with a different number of pins).  Power for logic and
Nixie drive (about +250V) was provided through the cable, as well as all
of the other signals.

I believe Brent Hilpert has derived schematics of the display, the
keyboard, and pinout of the connector of one of the Model 320K
keyboard/display units, which is has online on his website.  
Check out:

By the way, early design 300-series calculator keyboard/display units
had cast metal cabinets.  Later, the design was changed to a heavy
cast-plastic cabinet for cost savings.

Rick Bensene
The Old Calculator Museum


> -----Original Message-----
> From: cctalk-bounces at 
> [mailto:cctalk-bounces at] On Behalf Of Paul Heller
> Sent: Thursday, September 06, 2007 5:47 AM
> To: General Discussion: On-Topic Posts Only
> Subject: Re: Wang 300 Calc
> On Sep 5, 2007, at 4:10 PM, Brent Hilpert wrote:
> >
> > I received two orphan 320K keyboard/display units a few 
> years ago (no 
> > logic unit).
> > Hobbled up a little level-shifting hardware and wrote some 
> calculator 
> > code to drive them from a SWTPC 6800 - in other words the 6800 was 
> > replacing the missing logic unit. Worked fine but the next 
> (intended) 
> > step was to redo it all in a microcontroller and stuff it 
> inside the 
> > 320K case so it would be a stand-alone 'modern' nixie desktop 
> > calculator. Would be kinda nice because the 320K KDUs have a small 
> > footprint compared to other nixie desktop calcs. There is 
> very little 
> > spare room in the case however. I think I anticipated it would be 
> > feasible if one used an external wal-wart (ugly) for the power 
> > transformer.
> > The KDUs are very well built: cast aluminum case, 
> micro-switches for 
> > the keys...
> Cool idea. Did you find the hardware interface documented 
> somewhere, or did you reverse engineer it somehow? Was it 
> something standard like RS-232?

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