74123 was one of the first TTLs I have encountered while beginning my adventure with digital electronics back in the 1980s. I remember that a learning set I bought had a circuit for a programmable digital lock using this chip. You'd program the lock using timed pulses and then you'd have to use the same timing pressing the unlock button while opening the lock. If you failed to time the unlock sequence properly, alarm would sound. I built it and it actually worked :-)
Can I just say how much I love this chip videos? I can't tell you how many times I've got a chip, looked at the datasheet and still not been able to work out what it's all about. I have seen it suggested that the 123 can be used as a TV sync separator for H sync and V sync, but I've not had much luck with it myself.
Most common use was to handle logic race conditions, or to provide a delayed edge to trigger a latch, because it was on the other clock edge, but also had setup and hold times which meant you could not just invert the clock to use it, but instead had to do more. Thus the 122 with all the input logic, to handle part of the decode as well, and then you had on the output the correct edge pulse to get the data latched correctly. Often used when mixing logic blocks from different families, like interfacing 6800 parts to the Z80 or 8080, and vice versa, where you needed certain delayed edges to get reliable data transfers. Yes kind of critical in getting RC values right, too little and you got garbage because the bus was not ready, too long and you also got garbage from the bus being released. Another use was pulse stretching, so that a pulse train could be decoded into data over say a serial link, to discriminate between the 2 different tone widths, of say early tape transfer methods, one half being used to generate a clock for input, and the other being used, together with the clock from the first, to determine if the input width was more or less than the the short period, so being able to tell them apart without needing a complex timing off the CPU. Just read the clock, and on change look at the other for the data bit.
I used the 74HCT221 variant for a pendulum clock beat analyzer using a microphone. It's the non-retriggerable version that ideally can't be fooled into lengthening the output pulse once triggered. The purpose was to create a one shot pulse that was unaffected by the following clicks, that occur in the escapement, after the initial sound. Also used active opamp filtering to try and eliminate false triggering from ambient noises. This all fed into a Microchip PIC (16f88 or 16f628 likely) using the capture interrupt for microsecond resolution on timing. The output from the PIC chip was standard serial data the was processed by a Visual BASIC program on a PC. This project was started almost 20 years ago, hence a serial interface to something powerful enough to generate a graphical view of the beat. Active opamp audio filters are amazing. I used a couple of stages up front, to amplify and bandpass filter the input. You could make fairly loud ambient noise, yet the active filtering really attenuated it, preventing most accidental triggers. Fun stuff. :)
Interesting... here we used a lot 4538 cmos or later 74HC4538. My experience is to use real high quality caps on those circuits to get them long therm stable. In fact no ceramic caps at all - i i used the premium foil caps, the good old green WIMA MKP Polypropylene or MKC Polycarbonate caps.
I remember that back in the day 74121 was the most prevalent monostable used in schematics. Very similar to the 74122, only with fewer inputs - and the same problems.
The 122 was rather 1/2 of 123 than 121. Main difference between 121 and 122/123 was so called retriggerability - the output pulse could be extended if triggering happened before end of this pulse (with 122/123), while 121 could generate new pulse if previous one was finished (plus some dead time). As a result, pulses from 121 were always of the same length, while 122/123 was able to generate pulse with any length (but not shorter than time established by RC conditions).
About 3 years ago, I pulled out my old Imsai from storage and restored it. (I started watching your channel when you restored yours). A lot worked (I replaced the huge PS caps first off), but the front panel was flaky. The switches, like the deposit and deposit next pass thru those 74123's to get rid of the key bounce and to get a nice pulse, without triggering multiple inputs. Most of the old tech's I spoke with call these "one shots". I traced mine, found the offending parts (I had two bad) and swapped them in and it started working properly. But It was fun to understand what they did and that you could adjust the pulse timing with the RC network. I've have a digital scope, like your older one, and the ability to capture a "single trigger" sure made it easy!
I'm with you - whenever I saw a monostable in a digital circuit my first thought was that someone was too lazy to design it correctly and just punted. That's probably an overly harsh view and there are valid use-cases, but...
Don't bag the 123, it was the solution to a lot of problems which is easy to solve today much cheaper, but back then even a few extra counters instead was WAY more expensive!
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74123 was one of the first TTLs I have encountered while beginning my adventure with digital electronics back in the 1980s. I remember that a learning set I bought had a circuit for a programmable digital lock using this chip. You'd program the lock using timed pulses and then you'd have to use the same timing pressing the unlock button while opening the lock. If you failed to time the unlock sequence properly, alarm would sound. I built it and it actually worked :-)
Might need to drop in a bypass capacitor or two on the power leads a bit closer to your chip. (grin)
A lovely production, as always.
"What does a '123 do?"
It fills up a chip drawer...
At least in my case, got waaay too many of them.
Cheers,
Great video, I enjoy the “chip of the day” series. I appreciate your detailed explanations of the IC’s and how to hook them up. 👍🏻
Can I just say how much I love this chip videos? I can't tell you how many times I've got a chip, looked at the datasheet and still not been able to work out what it's all about. I have seen it suggested that the 123 can be used as a TV sync separator for H sync and V sync, but I've not had much luck with it myself.
Most common use was to handle logic race conditions, or to provide a delayed edge to trigger a latch, because it was on the other clock edge, but also had setup and hold times which meant you could not just invert the clock to use it, but instead had to do more. Thus the 122 with all the input logic, to handle part of the decode as well, and then you had on the output the correct edge pulse to get the data latched correctly. Often used when mixing logic blocks from different families, like interfacing 6800 parts to the Z80 or 8080, and vice versa, where you needed certain delayed edges to get reliable data transfers. Yes kind of critical in getting RC values right, too little and you got garbage because the bus was not ready, too long and you also got garbage from the bus being released.
Another use was pulse stretching, so that a pulse train could be decoded into data over say a serial link, to discriminate between the 2 different tone widths, of say early tape transfer methods, one half being used to generate a clock for input, and the other being used, together with the clock from the first, to determine if the input width was more or less than the the short period, so being able to tell them apart without needing a complex timing off the CPU. Just read the clock, and on change look at the other for the data bit.
I used the 74HCT221 variant for a pendulum clock beat analyzer using a microphone. It's the non-retriggerable version that ideally can't be fooled into lengthening the output pulse once triggered.
The purpose was to create a one shot pulse that was unaffected by the following clicks, that occur in the escapement, after the initial sound. Also used active opamp filtering to try and eliminate false triggering from ambient noises. This all fed into a Microchip PIC (16f88 or 16f628 likely) using the capture interrupt for microsecond resolution on timing. The output from the PIC chip was standard serial data the was processed by a Visual BASIC program on a PC.
This project was started almost 20 years ago, hence a serial interface to something powerful enough to generate a graphical view of the beat.
Active opamp audio filters are amazing. I used a couple of stages up front, to amplify and bandpass filter the input. You could make fairly loud ambient noise, yet the active filtering really attenuated it, preventing most accidental triggers. Fun stuff. :)
Interesting... here we used a lot 4538 cmos or later 74HC4538. My experience is to use real high quality caps on those circuits to get them long therm stable. In fact no ceramic caps at all - i i used the premium foil caps, the good old green WIMA MKP Polypropylene or MKC Polycarbonate caps.
I remember that back in the day 74121 was the most prevalent monostable used in schematics. Very similar to the 74122, only with fewer inputs - and the same problems.
The 122 was rather 1/2 of 123 than 121. Main difference between 121 and 122/123 was so called retriggerability - the output pulse could be extended if triggering happened before end of this pulse (with 122/123), while 121 could generate new pulse if previous one was finished (plus some dead time). As a result, pulses from 121 were always of the same length, while 122/123 was able to generate pulse with any length (but not shorter than time established by RC conditions).
About 3 years ago, I pulled out my old Imsai from storage and restored it. (I started watching your channel when you restored yours). A lot worked (I replaced the huge PS caps first off), but the front panel was flaky. The switches, like the deposit and deposit next pass thru those 74123's to get rid of the key bounce and to get a nice pulse, without triggering multiple inputs. Most of the old tech's I spoke with call these "one shots". I traced mine, found the offending parts (I had two bad) and swapped them in and it started working properly. But It was fun to understand what they did and that you could adjust the pulse timing with the RC network. I've have a digital scope, like your older one, and the ability to capture a "single trigger" sure made it easy!
I always enjoy these Chip vids. :)
The 74123 Makes a great pulse detector for a logic probe
The timebase sweep generator of my oscilloscope runs on chips like these!
I love old logic ics. I'm going to go see if I have that one. Thanks man.
you should put that pulse into a 10cm delay line and get a 1 gig oscillator.
Is it possible to generate 5ns pulse width with modern cmos chips?
I'm with you - whenever I saw a monostable in a digital circuit my first thought was that someone was too lazy to design it correctly and just punted. That's probably an overly harsh view and there are valid use-cases, but...
so this ic, puts out a square pulse instead of a discharge pulse, otherwise u can use just 1 capacitor.
would you do an experiments with the CD4046 (Doepfer eurorack PLL module video by MylarMelodies to learn)?
th-cam.com/video/P8agoshy3H4/w-d-xo.html
Creative video, like it, thanks :)
Thanks! This is the chip I requested... This is a chip you "love to hate"!
Don't bag the 123, it was the solution to a lot of problems which is easy to solve today much cheaper, but back then even a few extra counters instead was WAY more expensive!