5a-75B turning outputs into inputs? also 5a-75e about chubby75 HOT 20 OPEN

I came up with these flex boards ordered from OSH Park: https://github.com/Disasm/hc245t-bypass
With minor rework they can be soldered with a regular sodering iron and some flux.

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My current plan is to have some PCBs produced to replace the IC and just bridge the connection.

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I have documented my modifications here: https://zeromips.org/posts/2022-05-29-5a-75b
Involves flex PCB as well as SN74CBT3245APW.

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@tomverbeure The picture above is under the Creative Commons Attribution-NonCommercial License

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A parasitic diode is formed between the bus and VCC terminals.Therefore, the VHC245 cannot be used to interface 5 V to 3 Vsystems directly.

Datasheet

Although there is a 33 Ohm resistor between the connector and the IC, I wouldn't really trust it to not put 5V on my 3.3V rail.

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@cyber-murmel do you have the design somewhere? I was thinking of ordering some pcbs.

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@FFY00 The problem with this particular design was that fabs generally don't accept castellated holes that small.

But I recently got around to design and install another version of this idea. This time with three plated through holes per pad to wick in / wick through the solder.
just didn't come around to test it yet :/

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Hum, that might be a little bit difficult to line up. Let me know when you give it a try. Right now I think I can live with hand soldered jumpers on one or two chips.

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Most of the holes aren't clogged from the fab. I found it easy to align them by looking through the holes and having the pads of the footprint behind it create a parallax effect. Like when you line up a stencil with a PCB.

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I also found, that a big single-flat soldering tip on powerful iron with lots of flux is easier to use, than hot air.

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@cyber-murmel Hey cyber-murmel,
do you mind if I use that picture that you posted above in a presentation about FPGA board hacking?
Tom

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Sweet, just ordered a set, thanks.

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Also ordered, many thanks 😊!

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Thanks, I guess one just needs to not forget clicking "Flex" checkbox on ordering page (?)

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@fanoush yes! Non-flex boards will be useless.

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Nice! Just ordered a bunch of them as well.

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So just to undestand this correctly: with these bridges the inputs are directly wired to the fpga, thus they are only 3.3V tolerant?

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@jadafi Exactly.

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I might have found a way to get 5V tolerant inputs. I documented it on my github: https://github.com/kittennbfive/5A-75B-tools/tree/master/I_want_inputs

TL;DR: Replace (some of) the buffers with SN74LVC245APWR with some surgery to modify 2 connections on each IC.

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Speaking of IC replacement hacks, there is another option that involves SN74CBT3245APW bidirectional FET switches: https://twitter.com/Claude1079/status/1231194849350647808
Seems like it also requires additional hacks, but looks relatively easy.

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