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74123 monostable multivibrator

Mike_Z

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Over the weekend I found a Texas Instruments document "Designing with the SN54/74LS123" . It talks about how to use and what's in the chip. I use a dozen or so of these in my 8080. Mostly for the front panel controls producing pulse signals for various functions. Anyway, the first figure snagged my attention. This figure displayed the two mono's, connected up with an RC network, but what caught my attention was that the Cext terminal was grounded. I have never done this and have not experienced any trouble without this ground. So I combed over the document trying to find out why TI wants this terminal grounded. The only mention I found was, was "NOTE: For best results, system ground should be applied to the Cext terminals." and that's all I could find. The document does have a detailed internal circuit diagram of the chip. The Cext terminal is connected to internal ground, yet the circuit diagram omits the +5 pin 16 and System Ground pin 8 from the diagram. Just cause, I'm going to ground pins 6 and 14 on all my 74123's. Has anyone else had any experience with this?
Thanks
Mike
 

dave_m

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Has anyone else had any experience with this?

The 74121 and 123 one-shots are extremely noise sensitive. I have seen signals, not having anything to do with the circuit, but which are routed close by on the PCB that would trigger the thing. Blinking lights are possibly the only application they should be used, never for critical timing applications.
 
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Mike_Z

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Well....... as I said, I've never had any trouble with the mono's over the last 30+ years. It's just that I have never seen anywhere where it was implied, suggested or otherwise stated that the Cext terminal needs to be grounded.
Mike
 

MikeS

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I never cared much for the 123--when I had to use a dual one-shot, I'd usually grab the 9602. The 221 is somewhat better.
Big fan of the 555/556/558 myself...

But the '121 and '123 do have a rep (deserved or not) for instability and noise sensitivity; nevertheless I see them working just fine in quite a few commercial circuits.

BTW, I've got around 1000 '123s if anybody needs any... ;-)
 
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Chuck(G)

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Mike, I don't think I'd ever use a 555 to generate 100 nsec. pulses... 9602s were generally more stable and less susceptible to noise than the 123.

As usual, it depends upon what you want to do with it. There are many cases where an 8-pin MCU will do a job better than an 8-pin 555.
 

MikeS

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Mike, I don't think I'd ever use a 555 to generate 100 nsec. pulses... 9602s were generally more stable and less susceptible to noise than the 123.
Pshaw! Real men generate millisecond pulses or larger, none of this micro/nano/pico pussyfooting...
 

dave_m

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Well....... as I said, I've never had any trouble with the mono's over the last 30+ years. It's just that I have never seen anywhere where it was implied, suggested or otherwise stated that the Cext terminal needs to be grounded.
Mike

I think this note was added after many complaints about the chip's lack of noise immunity. Doing so provides a better copper path to ground than just relying on the internal ground connection. I looked at an old TI spec sheet (revised 1988 ) and it's a little more specific: instead of "For best results, system ground should be applied..." it says "For maximum noise immunity, system ground should be applied..."
-Dave
 
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