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Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Hey all!
I'm planning on making a nixie tube clock. I've got 4 of 74HC147 (decimal to BCD) converters, got 10 CD4017 Decade counters on the way, 5 CD4013 Type D flip flops (for dividing the 2 hz output of the 4060 to 1 hz), the crystals for timing, a bunch of 4060's (for making the 2 hz oscillator) as well as the 74141 nixie tube drivers.
Now, my question is, I'm technically mixing logic here... the only thing that I'm curious about is driving the 74141 inputs with the 74HC147s. 74141 is TTL, while the 74HC147 is CMOS; max supply for it is 6 volts... so everything else would have to be 5 volts. I can't figure out what the 74141 uses.
Will there be any complications as a result of mixing logic? Or would it be fine? I'm kind of noobish at logic stuff, so thanks for the help by pointing out any grave errors...
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The logic side of the 74141 will run fine off 5V, and its inputs should work fine with 74HC CMOS outputs.
The only time you need to be careful is when connecting an output of a TTL chip to an input of a CMOS one. The TTL outputs aren't guaranteed to go any higher than about 2.5V, which isn't enough to reliably register as "high" in a 74HC input.
In this case, you should use 74HCT series instead of 74HC.
And, I can't remember if 4000 series outputs have enough Oomph to pull a TTL input low. The current output of 4000 series is pretty measly when running off 5V.
If you're lucky, you may find a copy of Don Lancaster's "TTL Cookbook" and "CMOS Cookbook" in your local charity shop.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
If you ever need TTL to HC CMOS then run the HC CMOS chip at around 3.3 V, then it will be fairly voltage compatible with TTL. Another solution is to use the HCT version of the chip that is voltage compatible at 5 V. Yet another solution is to add a pullup on the TTL output.
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
If I’ve got this right your starting with a decimal counter 4017 then converting it into bcd 74hc147 then using a bcd to decimal convertor 74141, to convert it back to decimal again? Though at a higher voltage to drive the tubes.
Couldn’t you start with a BCD counter like the 74hc390 or similar? Or use lots of discrete Hv transistors from the outputs of like a HC4017.
Most of your cmos ic’s should have a HC equivalent, so you might be better off trying to keep to using mostly the same logic family. I have mixed HC and TTL running of 5v when I didn’t have the right IC’s and seemed to work ok, but it’s probably not guaranteed in every situation.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
TTL inputs source current, which has to be sunk by the driving stage. The question for CMOS driving TTL is fan-in, how many inputs can one output drive.
Old original steam-driven TTL has an input current of 1.6mA max, for newer TTL families it's lower, 0.4mA for LS, 0.6mA for F.
HCMOS run at 5v has a usable fanout into TTL. Although its fanout into CMOS, which takes the sqrt(buggerall) input current is more or less infinite at DC, it still needs a bit of output oomph to charge the capacitances that kick around everywhere in a reasonable time, so even your cooking-grade HCMOS will sink 3.2mA while meeting a TTL logic 0 level of 0.4v, so being able to drive at least 2 standard TTL inputs. It was being introduced when TTL was very much still the logic workhorse, so it had to be compatible to penetrate the market. CMOS buffer types can be had which go to 24mA.
IIRC, 15v cmos run at 5v still acheives 1.6mA sink, but only for the pure buffer functions, 4049 and 4050 inverting and non-inverting respectively. So a fanout of 1 using a buffer to interface it is rather more of a pain. Obviously 15v cmos run at 5v will fanout into HCMOS very easily, as long as you have no need for speed.
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CMOS to TTL
