Multiplexing Nixies

Dwight wrote: Multiplexing Nixies was extremely common in all but the earliest electronic calculators. The early machines (Sharp Compet 10, 15, 20, 21, 30[and Facit/Addo equivalents]; early Sumlock/ANITA machines; the Wang LOCI 1/2; and a few others) used discrete drivers per Nixie tube, but the cost of such circuitry (11 transistors per tube [digits plus decimal point], along with diode arrays to do BCD to decimal conversion for each tube) made the machines quite expensive with very high component counts. Multiplexing had two major advantages. The first was that the driver circuitry was MUCH less expensive. One BCD to decimal converter, 11 transistors for driving the anodes (digit 0-9 & decimal point), and as many transistors as there are tubes for switching the cathodes. Along with the transistors, there was usually a series of RC networks, or sometime small-signal transformers (Canon liked transformers) involved in the cathode switching. The other major benefit of multiplexing was that the architecture of most electronic calculators is bit-serial. This was done because the speed of parallel math is not necessary on a simple calculator, and that the ALU circuitry was much less complex. Also, most of the machines had a memory that was bit serial, such as shift registers or magnetostrictive delay lines (and in at least two known cases, magnetic drums). Those that had magnetic core memory, or stored charge capacitive memory were generally addressed in a bit-serial fashion. The bit serial nature of the logic made multiplexing a whole lot easier, because the bits are already circulating around in the machine, and this architecture fit very will with multiplexing. Typically, there would be a four-bit shift register that would accumulate a BCD digit, and this register would be fed to the BCD-to-Decimal converter, which would feed the anode drivers. A separate shift register or counter would step through the number of digits there are in the display, which would drive the cathode drivers. Everything was synchronized by phases of the master clock circuitry. The result was a much simpler design in terms of parts count (which translated directly to lower cost and higher profits), and also smaller, lighter, and more reliable machines. In terms of failure rates, I have yet (after many years of collecting) to find a multiplexed Nixie calculator in which a Nixie tube has failed. It's relatively common to find failed anode or cathode driver transistor failures, which result in either a single Nixie that refuses to light, or a single numeric digit (or decimal point) won't display on any of the tubes. In the case of a cathode driver failing, many might think the tube is bad. In every single case I've had, replacing the driver transistor(s) brings the tube back to life. Remember, calculators were typically turned on in the morning, ran all day long, and shut off when people went home for the day. So, the number of runtime hours on the tubes by the time I've gotten my hands on a machine is very long. I do believe that Nixies made for direct drive have some different characteristics than those made for multiplexed drive. I do not know specifically, but recall reading somewhere that the gas mixture in Nixies made for multiplexing is somewhat different, to allow for faster switch-on/off times. I seem to recall that there was a trace amount of mercury added to the Neon gas mixture. I also do know that the switching times for mux'd Nixies were rather strict. Violate the duty cycle and rise/fall times and current levels, and the tubes would indeed suffer a reduced lifetime. Calculator designers took all of this into account, and designed the drive circuitry very carefully to assure maximum life for the Nixie tubes. The common problem with old Nixie tubes (multiplexed or not) is long warm-up time. Some of the old machines in the museum take about 30 seconds to a minute before all of the tubes light up. This is generally caused by slight contamination of the gas in the tube by the anodes slowly eroding, or by molecular-level leakage through the glass envelope over very long periods of time, reducing the gas concentration in the tube. But, once the tube "warms up", it runs fine. This is part of the reason why I power up each calculator in the museum at least once a month and run it for at least a half hour, partly to exercise the tubes, and also to keep the power supply filter capacitors from malforming. I generally do a few machines per day, and over the course of a month, I've pretty much run through all of them. I put a priority on the Nixie and CRT display machines, as they are the most likely candidates for problems, then the VF tube calculators, followed by LED's, then the calculators (like the early Canon 161 and 130S) with incandescent displays, and lastly, the early LCD calculators. So, in summary, Nixies multiplex, and do so just fine. However, some Nixies aren't designed for it, and multiplexing these tubes may shorten their liftimes. But, the tubes that are designed for multiplexing can have extraordinarily long lifetimes (some of the Nixies tubes in machines in the museum are nearing 40 years old, and still work beautifully). Rick Bensene The Old Calculator Web Museum http://oldcalculatormuseum.com