# Simple as a diode?

Discussion in 'General Electronics Chat' started by s_mack, Feb 11, 2013.

1. ### s_mack Thread Starter Active Member

Dec 17, 2011
198
6
Hi. I'm trying to conceptualize how to connect 2 or more of three "devices", A B and C, utilizing an identical cable between any two pairs, when the requirements of one particular pin - let's call it Pin X - are different depending on the pair (the rest serve identical purposes regardless of the pairing, and so can be ignored).

A is always in the equation. It may be connected as A to B, or A to C, or daisy chained as A to B, B to C (B has two headers) but never A to C, C to B.

On A, Pin X is connected to the output of a 5v regulator. When connected to B (on header 1), that wire provides power to B... so that's straight forward.

On B, Pin X (on header 2) is a uC input... expecting a signal from C. That too is straight forward. (to be clear, Pin X on header 1 is completely independent from Pin X on header 2)

So I have no problem with A to B or A to B to C. My issue is with A to C.

On C, Pin X is connected to either ground or C's VCC by way of a switch. So when connected with B, that sends a high or low to B's uC. But when connected to A... I want it completely ignored. I don't want A's +5v output frying anything and I don't want C's potential connection to ground shorting anything!

Possible?

I'd draw a schematic, but honestly I don't even know how to represent it graphically. If I did, I probably wouldn't be having an issue conceptualizing it

I'm wondering if its as simple as having a diode on both A and C? I suppose my biggest concern is what happens when C's switch has its Pin X connected to ground and A's +5v is connected to it.

I know my thoughts are a bit of a mess here. I hope someone can figure out what I'm trying to say, and throw it back to me in a language that triggers my brain into seeing the problem more clearly I'm betting its either "impossible" or "simple" lol

Thanks

- Steven

Apr 14, 2005
7,018
682
Sorry-

3. ### s_mack Thread Starter Active Member

Dec 17, 2011
198
6
I don't understand the reference.

4. ### Ron H AAC Fanatic!

Apr 14, 2005
7,018
682
Yeah, that was an inappropriate reply. "Snow job" implies an attempt to deceive. I didn't think you were in any way trying to deceive. I just meant that I was totally confused.

5. ### SPQR Member

Nov 4, 2011
379
49
I'm confused.

What kind of device is it?

6. ### Ron H AAC Fanatic!

Apr 14, 2005
7,018
682
You made it look like a riddle.

7. ### s_mack Thread Starter Active Member

Dec 17, 2011
198
6
Lol. Yeah, no intention to deceive or confuse.

I didn't really think it mattered what kind of device we're talking about. And its a bit hard to explain, but here goes:

"A" is essentially a purpose-built ISP programmer who's full time job it is to update "D" - an unmentioned 4th device in this system that really isn't part of the equation for these purposes. "B" and "C" are add-on devices for "D". Each has an AVR chip in it and each has to be updated periodically. All are encased in an enclosure that isn't trivial to access, which is why there's a need for "A" at all. So my concept is to utilize "A" to also program "B" and/or "C". Of course, only one device may be connected at a time to the ISP programmer. I have everything worked out, except the one pin. That pin is used to send a signal from C to B to trigger logic that transfers the programming signals from A, THROUGH B, and on to C. The absence of that signal means A is to program B.

At the heart of the "riddle" is that these devices can be used in the combinations described earlier. Not every end user has B or C or either (everyone has A and D). Logic onboard A determines if programming is for D or "other". If "other" is just B, its no big deal. If "other" is C via B... I've got that figured out too (that's scenario I attached). My only issue is if someone has C and no B... because the same pin that would send the signal from C to B to initiate the trigger... has instead a +5v source from A (because that same wire is used to power B... if B is there).

I fully appreciate the complication here. But its OK... just consider this self-therapy Typing it out and asking for help is, in itself, forcing my brain to work it out . And I'm pretty sure I have it figured out now.

I think it pretty well comes down to the attached. Scenario II is the one I was having difficulty with. But now that I've represented it graphically, I think its clear that a double diode should do the trick. As far as pin x is concerned, A and C should be oblivious to each other, right? And that's what I want. In that scenario, I'd like it to behave as though that wire isn't even there (which is an obvious method, but I'd prefer to not have end users forced to modify the cable)

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