On Friday, March 31, 2017 at 4:42:53 AM UTC-7, Chuck wrote: > > 360 ohms is way too little resistance for *any* sort of > TTL pullup resistor! That apparently got confused with > the standard *pull down* resistor used to keep an unattended > TTL input *low*. 360 ohms for a TTL pulldown is just right. > > 2.2k ohms is the standard value to use as a TTL pullup. > That goes for regular old 7400 series TTL, as well as 74LS > series. > > The old military practice was to always use 1k ohm as the > standard TTL pullup resistor value. Note that even when using > that 1k pullup resistance, that all we are asking of any TTL > gate which is pulled up by this, is to be able to provide 5 > milliamperes > in order for for the output to go low. > > Even that value of 1k ohm would work ok with either plain 7400 or > 74LS. > > 2.2k is much more like it, however. > At that value, all we are asking is 2.3 mA, and that works fine. > > 360 ohms is just simply too close to being a piece of wire!! > > (This is all explained in great detail on page 12 of Don Lancaster's > famous TTL Cookbook) > > I think he's stuck with the low value resistors, because he's making Schmitt triggered gates out of 7417s. The feed back resistor has to work both as a pulldown, and pullup. As noted, TTL source and sink, are very asymmetrical.
This is one, of several reasons, I prefer, 4000 series CMOS over 74 TTL, when it comes to older logic families. CMOS is symmetrical. It has high impedance inputs, and you're not stuck with a 5V supply. On my "new" thermometer kit, I run the CMOS on 12V. Here is the set switch scheme on my old nixie clock board: <https://lh3.googleusercontent.com/-VazC16Fj59o/WN5MVNLqDjI/AAAAAAAAYlk/NTixbY4pxCcpFlfyD839n1grkagyRzd2wCLcB/s1600/3N_Clock.jpg> The "seconds-to-minutes" and "minutes-to-hours" pulses are passed thru 1000pf caps, feeding 4.7K loads, that normally sit at 5V. It has to use a 5V supply due to the 74141 drivers, also used. Short pulses are seen at the respective inputs, for minute, and hour, "bumps". Setting is done thru 4093 Schmitt gates, which are made "open drain", by using 1N914 diodes, in their output legs. Since its 4000 CMOS, a logic-0 is anything under 1/3 supply, or ~1.5V or less. A logic-1 is anything over 2/3 supply (>3.5V). The 0.7V diode drop is a non-issue, here. Punching the "hours" or "minute" button, increases the count by one, per depression. No goofy accelerated clock scheme. This would be much more difficult to implement in TTL. -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/2f6eb5a9-3e84-46f3-8f0f-874e96776878%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
