Please don't bother asking "why" or trying to suggest a different method entirely... I just want to know if this is possible:
Can I connect (permanently) 1 ISP programmer via MISO/MOSI/SCK/RST to multiple targets if I know for certain that only the intended target will be powered at the time of programming?
Clearly the answer is "no" as I have it wired now, because it doesn't work. The powered down targets still manage to interfere because I have to disconnect the serial lines from the "dead" targets before the live one will program.
I can isolate using a variety of means, EXCEPT... I have no ability to change the circuit between the ISP and target #1. They are hardwired. Please consider that an immovable object. They are connected and there's nothing I can do about that or insert between them. However, I have full control over everything "down the line" (ie the other targets). So through use of p-FETs (and i'm sure logic buffer would be a better solution) I have successfully made it so I can program target #1 without a problem (by hi-Z the others)... but, as just stated, I can't isolate #1 from the rest and so far everything I've thought of ends up in a circular argument.
Is there some trick I can do "down the line" so that the unpowered #1 isn't interfering with the programming of #n? Some use of pull-ups or a fancy dancy IC i've never heard?
Probably not, but thought I'd ask before giving up.
I'm sure someone's going to ask for a schematic. You all know how a connection between ISP and target look, so now picture those lines branching off multiple times. I have full design control over everything past #1 and consider it patently impossible to change/insert/even touch anything from #1 to the ISP.