New H8 Front Panel 74LS123 issue.
Norberto Collado
While doing final verification of the new H8 front panel, I noticed that the 74LS123 is not firing as expected to protect the display from burning out a single digit. The input to the IC is fine, but the output is always high on pin 5. Also I did replace the 74LS123 and no changes.
Any ideas on what might be the issue?
Per Heath the cap should be “.15uf Mylar”. I ordered the following from Mouser:
Termination Style: | Radial | |
Product: | General Film Capacitors | |
Dielectric: | Polyester | |
Capacitance: | 0.15 uF | |
Voltage Rating DC: | 250 VDC | |
Voltage Rating AC: | 160 VAC |
Heath original cap picture:
Thanks,
Norberto
Norberto Collado
I just found that the issue is with the 74LS123 type. The 74LS123 will not work on such configuration.
Heath is listing a 74123. I took the 74123 out of my old H8 FP and put it on my new FP board and it is working fine now. Put the 74LS123 on the old board and it doesn’t work.
Thanks,
Norberto
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Norberto Collado
After probing around the 74LS123 and 74123, the 74123 does not ground pin 6 as I can measure around 1 volt on such pin. The 74LS123 does ground pin 6 internally. I think what I need to do is recalculate the value of R103 “40K” for a value that can provide a output pulse of 1.5ms high and .5ms low. Current circuit with such values cannot provide such timing when using the 74LS123.
More investigation in progress.
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Steven Feinsmith
I recalled about 74123 and 74LS123 also some others as "123" as a retriggerable monostable multivibrator family. The important thing is the capacitor, as you mentioned in the Heathkit manual. I believe this information will help you.
https://www.ti.com/lit/an/sdla006a/sdla006a.pdf?ts=1620454796856&ref_url=https%253A%252F%252Fwww.google.de%252F
I look forward to hearing from you soon.
GN,
Steven
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Norberto Collado
What works:
A 74123: C=.15uf, R=40K
A 74LS123: C=.15uf, R=30K
A 74LS123: C=0.1uf, R=40K
A 74LS123: C=0.1uf, R=50K
Doesn’t works:
When using a 74LS123: C=.15uf, R=40K – This doesn’t work.
That explains on why the LED’s were very bright. Once I got the circuit to work, the brightness lowered to a safe level. For those using a 74LS123 on such configuration, just change the capacitor to a 0.1uf to protect the LED’s. I left an LED digit idle in such state, and the LED and transistors got very hot. Now this LED lost some brightness when compare to the other LEDs. So it got some level of damage.
I will update the schematic to use the 74LS123 with a 0.1uf (common capacitor) along with a 50K resistor as this gives me the brightness I need. I will order a 50K resistor to test it out properly. Right now I’m running this circuit without any issues, using the 0.1uf (104) and the 40K resistor.
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Norberto Collado
Thanks Steven! Details on the other email.
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Norberto Collado
50K ohms resistor is not common, so I ordered 47K to test instead.
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Norberto Collado
The equation are different between each type of IC for Tw.
74123:
74LS123:
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Norberto Collado
Last, I need to check the layout for these requirements as pin 6 (Cext) is not grounded to the common ground.
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Robert Demkowicz
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Robert Demkowicz
//rjd
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Norberto Collado
Thanks Robert!
From: "se...@googlegroups.com" <se...@googlegroups.com> on behalf of Robert Demkowicz <r.j.de...@gmail.com>
Reply-To: "se...@googlegroups.com" <se...@googlegroups.com>
Date: Sunday, May 9, 2021 at 5:16 AM
To: "se...@googlegroups.com" <se...@googlegroups.com>
Subject: Re: [sebhc] New H8 Front Panel 74LS123 issue.
I believe it should be pins 6 and 7. It looks like the copy from Heathkit is hard to tell 6 from 5
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Steven Hirsch
> While doing final verification of the new H8 front panel, I noticed that
> the 74LS123 is not firing as expected to protect the display from burning
> out a single digit. The input to the IC is fine, but the output is always
> high on pin 5. Also I did replace the 74LS123 and no changes.
>
recall, we saw the same issue when I built my Z37-H89 controller. I didn't
have a 74LS123 on hand and (from memory) plugged in a 74S123 borrowed from the
Heath Z37 controller. It didn't work and neither of us could fathom why. I
ordered an LS part and all was well.
Norberto Collado
I did follow what Les did on such circuit, but it needed a 74123 instead of the 74LS123. That was what HEATH did specify.
I will re-layout such circuit per specs and will hard wired it to only support the 74LS123.
Overall the 74LS123's are tricky to get them operational @ 100% on power-on. That is why I do spend a lot of time testing and retesting. It needs to work properly every time I power up the system.
So far the circuit is working better now with last night changes after recalculating values.
Thanks,
Norberto
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norberto.collado koyado.com
Sent: Sunday, May 9, 2021 2:10 PM
To: se...@googlegroups.com <se...@googlegroups.com>
Subject: Re: [sebhc] New H8 Front Panel 74LS123 issue.
Steven Feinsmith
Mouser Part # | Vishay / Dale | Metal Film Resistors - Through Hole 1/8watt 50Kohms .1% 10ppm |
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Norberto Collado
Thanks Steven for your feedback. I’m tempted to use two resistors to fine tune it to the decimal point to get 1.5ms. Here are the results with a single resistor.
74LS123:
With Rx = 40K and Cx = .1uf (104), I get a pulse of 1.42ms.
With Rx = 47K and Cx = .1uf (104), I get a pulse of 1.67ms.
This pulse controls the LED “On” time to prevent LED digit burn out if CPU fails to refresh the Front Panel Display. Also the duration of the pulse controls the LED brightness. At 1.67ms the brightness is great.
I will continue testing and will update schematics to use the 47K resistor along with a .1uf capacitor. In addition, Terry S. is sending me some “74AHCT123” to test them with such values as he gets great results with this part.
74123:
Heath Front Panel with 74123 IC, Rx=40K, Cx=.15uf. Pulse is 1.87ms. Pictures below for each config.
Thanks,
Norberto
74LS123: Rx = 47K and Cx = .1uf = 1.67ms 74LS123: Rx = 40K and Cx = .1uf (104) = 1.42ms.
74123: Rx=40K, Cx=.15uf = 1.87ms. This is out of specs as it should be 1.5ms. The ”On” time is pretty close to 2ms (1.87ms) which might explain why it fails with the 74LS123, which keeps the output “High” all the time.
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Norberto Collado
Steven,
I could not resist and with a two resistor setup (40K + 3.3K), I get 1.5ms per Heath specs (43.3K).
74LS123: Rx=43.3K, Cx = 0.1uf (104) = 1.5ms. I will update the schematics to use a 40K + 3.3K to have more margin as this is the most important circuit that protects a single digit burn out if CPU get stuck for any reason.
Thanks,
Norberto
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Norberto Collado
43K is the final number.
Steven Feinsmith
- Jameco Part no.: 853791
- Manufacturer: Bourns
- Manufacturer no.: 3296W-1-503VP/LF
- 1: $1.49
10: $1.25
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Norberto Collado
For better adjustments based on the waveforms, it must be a 40K resistor in series with a 10K pot. As the capacitor value might vary, this makes more sense to add.
Norberto
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Norberto Collado
Received today the 43K 1% resistors and did some preliminary testing on the new Front Panel Board:
RX = 43K 1%, Cx = 0.1uf
First 74LS123: 1.53 ms
Second 74LS123: 1.56 ms
Third 74LS123: 1.54 ms
Heath 74123: 1.18 ms
First 74AHCT123: Should be here hopefully tomorrow.
Second 74AHCT123: Should be here hopefully tomorrow.
So far the 74LS123 with 43K and 0.1uf is the winner.
With layout changes it should be more accurate.
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Norberto Collado
Also, I’m going to recalculate Rx to use a .1uf cap for C105; “RUN” LED timing. This is to avoid having different/unique caps for the 74LS123.
Norberto
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Norberto Collado
Per specs the low section of the pulse should be 4.7ms for the “RUN” LED to be on.
Measured on Heath FP board (74123): 5.92ms
Measured on new FP board (74LS123): 11.3ms
Based on input timing signal (M1), I cannot use 0.1uf as it will never trigger. The 11.3ms should be fine as the LED is much brighter. No changes on components.
From: "se...@googlegroups.com" <se...@googlegroups.com> on behalf of Norberto Collado <norberto...@koyado.com>
Reply-To: "se...@googlegroups.com" <se...@googlegroups.com>
Date: Thursday, May 13, 2021 at 7:40 AM
To: "se...@googlegroups.com" <se...@googlegroups.com>
Subject: Re: [sebhc] New H8 Front Panel 74LS123 issue.
Also, I’m going to recalculate Rx to use a .1uf cap for C105; “RUN” LED timing. This is to avoid having different/unique caps for the 74LS123.
Norberto Collado
I tested 74ACHT123 & 741HC123 on current circuit values on the new FP. They both failed to produce an output pulse on both circuits.
In conclusion you cannot replace a 74123 or 74S123 or 74LS123 or 74ACHT123 or 741HC123, with a different XX123 IC. If you do that, then the Rx and Cx values will need to be recalculate again to make it work properly.
The New Front Panel will use a 74LS123 with the following values: Rx = 43K 1%, Cx = 0.1uf & Rx = 27K 1% and Cx =680pf.
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