Diodes

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gesari
Posts: 19
Joined: 11 Apr 2019, 07:15
Location: Valladolid, Spain
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Diodes

Post by gesari » 11 Apr 2019, 08:37

Hi,
I'm very pleased to find this nice and clever computer design. And well documented too.
In this regard I found in the schematic you're using germanium diodes for logic (at least for the AND gate, in decoding ROMs the diode model is not specified). This looks odd, and in fact, the videos and photos show diodes in the style of the common 1N4148, that, BTW, is perfectly ok for this use.
Please, confirm the diode model used in the kit.

Also I was a little worried about the delay of this diode-resistor logic, but I found it was no more than 20ns, more or less in the same order of the logic chips.

Regards

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marcelk
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Joined: 13 May 2018, 08:26

Re: Diodes

Post by marcelk » 11 Apr 2019, 11:23

In the design all signaling diodes are the same. We recommend 1N60P or BAT42 compatible diodes, especially when using 74LS logic. Dave Jones has used the more standard 1N4148 diodes for his 74HCT-based build, and that combination seems to work for him.

The consideration is voltage drop versus the noise margin in the logical low region (≤0.8V). Voltage drop for diodes depend on the current (lower is better in our case) and temperature (higher is better). [Edited: Speed is mostly determined by the pull-up resistors. L-to-H transitions are slow. But we get a little helping hand from the TTL levels here: ≥2.0V is sufficient to become H.]

P.S: I don't believe that 1N60P is a true germanium diode, despite what some datasheets suggest.

gesari
Posts: 19
Joined: 11 Apr 2019, 07:15
Location: Valladolid, Spain
Contact:

Re: Diodes

Post by gesari » 15 Apr 2019, 10:25

So they are Schottky barrier diodes. This makes more sense, because new germanium diodes are difficult to find (not in Digikey, nor Mouser...)

Just for comment: You can save one diode in the AND gate. Just remove one diode and place the resistor in ts place instead. This two component AND gate will have a longer H-to-L delay in its resistor input, but it will probably work in the Gigatron...

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