9206	2013-10-04 12:26:34	dnorbury	SN74121 Monostable multivibrator circuits
Message	Date	From	Subject
I recently got hold of some Texas Instruments SN74121 monostable multivibrator ICs and was surprised that the data sheet didn't show a single application circuit. It's an older part but still being actively sold so I would expect that a number of application circuits exist. But a search on the web has so far yielded nothing. Does anyone know of a source for any circuits? Thanks for any suggestions. Dave, KD6A
9207	2013-10-04 12:57:03	iv3gfn	Re: SN74121 Monostable multivibrator circuits
Try this link: http://www.utm.edu/staff/leeb/3b3.htm 73's IV3GFN
9208	2013-10-04 13:08:16	dnorbury	Re: SN74121 Monostable multivibrator circuits
Thanks, that's a good start!
9209	2013-10-04 13:18:36	Stewart@g3ysx.org...	Re: SN74121 Monostable multivibrator circuits
I remember on pretty much my first day as a logic designer being firmly told that mono-stables were the devil's children. I don't think I have used one in any application since. All they do is to give you more opportunities for metastability failures. You mileage may of course vary :) 73 Stewart/G3YSX Sent from my iPad On 4 Oct 2013, at 20:26, <dnorbury@triad.rr.com> wrote: I recently got hold of some Texas Instruments SN74121 monostable multivibrator ICs and was surprised that the data sheet didn't show a single application circuit. It's an older part but still being actively sold so I would expect that a number of application circuits exist. But a search on the web has so far yielded nothing. Does anyone know of a source for any circuits? Thanks for any suggestions. Dave, KD6A
9212	2013-10-04 14:00:57	dnorbury	Re: SN74121 Monostable multivibrator circuits
Stewart, "devil's children" - that's great! Maybe TI was aware of that and decided not to put any applications info in the data sheet, thinking that would discourage anyone from using them! Thanks for the comment, Dave, KD6A
9214	2013-10-05 10:12:48	kb1gmx	Re: SN74121 Monostable multivibrator circuits
Having endured the 74121 and the '123 in early designs (8008 and MITS Altair) I can say that they are a great cause for pain. The only circuits I found that made them palpable were those that had seriously n
9215	2013-10-05 13:36:33	AD7ZU	Re: SN74121 Monostable multivibrator circuits
The primary application of the 74121 is an educational tool to illustrate to digital engineers what could happen in the real world. Civil engineers were scared straight after watching the video of the Tacoma narrows bridge collapse, petroleum engineers were shown videos of the lake Peigneur drilling fiasco which sucked down about a dozen barges, digital engineers were assigned a 74121 lab problem. Randy AD7ZU
9216	2013-10-05 18:40:28	dnorbury	Re: SN74121 Monostable multivibrator circuits
The excitement around the 74121 continues to grow! As I continue to mess around with them I hope I can find at least some small redeeming qualities. Real world experience of digital engineers (which I would liked to have prior to buying the74121s and 74123s!) is not that all that encouraging. I spent much of my career as an RF and microwave component/subassembly design engineer so I dealt almost exclusively with the high frequency analog world. Needless to say, we had our own problems as well. 18 GHz signals can do some funny things! It just goes to show that a good sense of humor is important in everything - no matter the frequency! I appreciate your comments.
9217	2013-10-05 21:49:28	Ashhar Farhan	Re: SN74121 Monostable multivibrator circuits
There was another abomination called the analog delay line. I used it to time column and row address lines in DRAMs. That was almost another lifetime away... The 74121 and the analog line delay were both terribly unpredictable beasts. - f On 10/6/13, dnorbury@triad.rr.com <dnorbury@triad.rr.com> wrote: > > > The excitement around the 74121 continues to grow! As I continue to mess > around with them I hope I can find at least some small redeeming qualities. > > > > Real world experience of digital engineers (which I would liked to have > prior to buying the74121s and 74123s!) is not that all that encouraging. I > spent much of my career as an RF and microwave component/subassembly design > engineer so I dealt almost exclusively with the high frequency analog world. > Needless to say, we had our own problems as well. 18 GHz signals can do some > funny things! It just goes to show that a good sense of humor is important > in everything - no matter the frequency! > > > > I appreciate your comments. > > > >
9218	2013-10-06 03:32:17	Roelof Bakker	Re: SN74121 Monostable multivibrator circuits
Hello all, Thirty years ago I build a frequency counter with 74LSXX parts and used one and a half 74123's in the timing circuit to generate gate and reset pulses. It works like a charm as of today! So, I wonder what the problems are that were encountered using this part. 73, Roelof Bakker, pa0rdt
9222	2013-10-06 10:56:59	Dana Myers	Re: SN74121 Monostable multivibrator circuits

9230	2013-10-09 21:29:14	Stewart Bryant	Re: SN74121 Monostable multivibrator circuits
All frequency counters intrinsically have the metastability problems, but people live with it, and can generally ignore spurious results. A case where this might be a problem is in the measurement of oscillator stability since these measurements can take thousands of samples to make a cumulative measurement and run over hours and possibly days. The problem is this. If you provide an asynchronous input to a clocked digital circuit and the asynchronous edge coincides with the clock transition the input may be registered as 1, or 0, or it may oscillate between 1 and 0 for an indeterminate period. Not really a problem on an ordinary frequency counter, but potentially catastrophic in a computer. Thus logic engineers double or even triple rank asynchronous interfaces, and sometimes use special logic cells designed to be relatively impervious to the effect compared to the high density circuitry used in the fully synchronous domain. At DEC we has two types of 74x74, normal ones and expensive ones pre-tested for their metastability characteristics. 73 Stewart/G3YSX
9231	2013-10-09 22:48:41	Ashhar Farhan	Re: SN74121 Monostable multivibrator circuits
Sluightly off topic though, feeding an fm signal to the 74121 and xor-ing the input and output provides very linear fm demodulation. - farhan On 10/10/13, Stewart Bryant <stewart@g3ysx.org.uk> wrote: > All frequency counters intrinsically have the metastability > problems, but people live with it, and can generally ignore > spurious results. A case where this might be a problem is > in the measurement of oscillator stability since these > measurements can take thousands of samples to make > a cumulative measurement and run over hours and possibly days. > > The problem is this. If you provide an asynchronous input > to a clocked digital circuit and the asynchronous edge > coincides with the clock transition the input may be > registered as 1, or 0, or it may oscillate between > 1 and 0 for an indeterminate period. Not really a problem > on an ordinary frequency counter, but potentially > catastrophic in a computer. Thus logic engineers > double or even triple rank asynchronous interfaces, and > sometimes use special logic cells designed to be relatively > impervious to the effect compared to the high density > circuitry used in the fully synchronous domain. > > At DEC we has two types of 74x74, normal ones and expensive > ones pre-tested for their metastability characteristics. > > 73 > > Stewart/G3YSX > >