Nixie driver chip question

[Alan 'A.J.' Franzman]

> Date:    Sun, 19 Mar 2006 11:07:51 -0500
> From:    "Tom K3TVC" <tchesek at EPIX.NET>
>
<snip>
> I came across a website listing truth tables for the 7441, 7441A, 7441B,
> 74141 chips and there are differences. Is there anyone who has used a
> substitution for the 7441B or knows if the differences in these chips
> have any impact on these counters?

The differences are as follows:
The 74141 has protective zener diodes at its outputs (approx. 55-60 volts)
which 7441 lacks.
The 74141 has a poorly-thought-out blanking feature which is rarely used (and
even more rarely works as intended, due to the aforementioned zeners.)
To (attempt to) blank a 74141, input a binary code greater than 9 (1001 bin).
If your nixies and supply voltage are exactly right, the nixie will go out.
But what usually happens is the 60 volt zener drop is insufficient to
extinguish the nixie, and all digits light dimly at the same time, forming an
indecipherable blob. Many pieces of equipment using 74141s will often power up
with one or more digit driver modules in an illegal or indeterminate state and
present such a display until they are reset.

The 7441 does not blank. If a binary code greater than 9 is input to a 7441,
the result is dependent on the exact make/revision of the chip. Some will give
2 outputs active simultaneously, others will start counting again from zero
(i.e. 1010 bin = 0 dec, etc.), and others will give "8" for the evens and "9"
for the odds above 1001 bin. I do not know of any equipment designed to take
advantage of any of these odd states for illegal inputs to a 7441. Thus, I
don't know of any case where replacing a 7441 with a 74141 would cause a
malfunction.

The Russian K or KM 155ID1 chips are identical in function to the 74141,
except that the output transistors and protective zeners are rated about 100
volts - making the blanking function nearly always work correctly. This may be
a rare case where the Russian copy is actually better than the original.
However, there is a slight mechanical difference. Russian ICs and printed
circuits of the period were based on a 2.50 mm grid as opposed to our 0.1 inch
(2.54 mm) grid. This means that the legs of the IC may need to be tweaked
slightly to fit properly in a western socket or PCB. It's not usually a
problem with small ICS like these, but can be a major headache with larger
chips as the difference accumulates over longer rows of pins.

Best regards,

Alan "A.J." Franzman

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