Diode choice

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Arne
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Diode choice

Postby Arne » Fri Sep 13, 2019 9:49 am

Hi guys,

I'm working on a small add-on for the cartridge port and need to AND /ROM3 and /ROM4 to get one combined /CS (for an EPROM) whenever the 128KB address range is accessed.
So I did use two diodes and a resistor (In1/2 = /ROM3, /ROM4 and Out = /CS):
Image
It seems to work on a 1040STFM (I used the Diag SW 4.4) but not on a F030 (Diag SW 1.27b/1.25)
as it outputs EA Address Error messages to the screen.
So I connected the scope:
Image

Image

As you can see I used two different sets of diodes. RL is now down to 3K3 and I don't dare to decrease it further. Screenshots are taken with 3K3 as RL.
I think that the F030 problems arise from the slow rise time of the diodes.
So either use diodes with less capacitance or decrease RL.
So far I've selected BAV18, 1N914 and 1N4151 for further investigation but I'm really not sure if these are good choices.
Anyone with a hint which diodes (THT) may solve the problem?

Thanks, Arne
Image

tzok
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Re: Diode choice

Postby tzok » Fri Sep 13, 2019 12:02 pm

Try with BAT42 or BAT43, these are true Shottky diodes, while all your propositions are small signal switching silicon epitaxial planar diodes.

https://www.vishay.com/docs/85660/bat42.pdf

https://www.vishay.com/docs/81857/1n4148.pdf
https://www.vishay.com/docs/85523/1n4151.pdf
https://www.vishay.com/docs/85622/1n914.pdf

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Arne
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Joined: Thu Nov 01, 2007 10:01 am

Re: Diode choice

Postby Arne » Fri Sep 13, 2019 12:17 pm

Thanks a lot. Will order some and report.
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