Shift Register Port Expander, lsrio16 comp

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29 Nov 2021 10:13 #227880 by DoWerna
I have built a small control panel for my small "cheesemill" , it hangs on a second parallel port. To have enough inputs I connected the small TSHW parport board from talla83 and intigrated the lsrio component in the Hal.
github.com/talla83/tshw-parport

There are two 74hct597d soldered on the board.

unfortunately not all inputs work reliably.

Some buttons always work, some only intermittently,
the faulty inputs are distributed on both 74hct597d.

the board looks clean soldered.
I rewrote the configuration and connected the panel to the other parallel port, but it didn't change anything.

what else can it be?

panel and parallel port cable are shielded!
could it be a latency problem?


Grüße
Werner

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29 Nov 2021 11:06 #227882 by tommylight
Do you get latency warnings?
Also check if those chips have a capacitor near them (1 each), 1 to 10 microfards, should be wired to positive and negative of the chip power.

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29 Nov 2021 11:51 #227886 by DoWerna
no i have no latency warnings,

yes there are 2 small caps near to the chips.
i will check the values.

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29 Nov 2021 12:05 #227890 by tommylight
Ok, check the voltage feeding the chips, should be 5V +-5% (4.75-5.25V) for TTL chips while CMOS chips work with a wider range.
Also check the board for "cold solder joints", those cause a lot of headache.
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29 Nov 2021 14:06 #227899 by DoWerna
Perhaps the PCB is still too dirty with flux that creepage currents occur?


If it looks like a hardware problem, I should still have all the components to assemble a second board.

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29 Nov 2021 14:11 #227901 by tommylight
74hct597d
If i recall correctly, that is a CMOS version of the TTL chip, the very fast type, so it does work with 5V strictly but requires way less current to switch the inputs.
I would add some resistors to all inputs on the switch side first.

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29 Nov 2021 14:56 #227907 by DoWerna
hm
i think the t in hct597d  stands for ttl

i have 2 74hct597d on the board witch makes the troubble.
if i remember right i have also 2 74hct597t at home, but i was not able to google the difference

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29 Nov 2021 15:13 #227908 by tommylight
C is for CMOS, t should be for higher speed, the letter at the end should be for temperature range, but i might be off, been over 30 years since i did that daily.
None of that is important for this use case, they should work.
Do you have a TTL tester? It is very helpful and it is easily DIY.
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29 Nov 2021 17:45 #227931 by DoWerna
unfortunately i do not have a ttl tester
easily DIY? äähm how? :-)

the caps on the board have a capacity of 100nF

as chip at the moment i have 2 74HCT597D on the board.
i have also some pc74hct597t which I also ordered at that time

should I exchange the chips for others?



 

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29 Nov 2021 19:35 #227940 by tommylight
There is nothing to be gained by changing chips for this use case.
Here:
www.allaboutcircuits.com/projects/diy-to...our-own-logic-probe/
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