I recently picked up a cheap USB based ISP (in-system programmer) for Atmel AVRs. There are two variants of the programming socket that are used with these devices, a 10-pin and a 6-pin. Unfortunately my ISP only came with a 10-pin cable, whereas most of my devices use the 6-pin socket. It is pretty easy to construct a 10-to-6 pin adapter (the signals are the same, they're just physically arranged differently on the larger 10-pin connector) and I would like to make a 10-to-6-pin ribbon cable. What is the maximum length that I can make it? (It came with a fairly short cable, about 8 inches, and I would like to slightly lengthen this if possible, to 1 or maybe 2 feet.)
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1\$\begingroup\$ Don't know what device / programming type you have in mind but some of the newer methods like PDI can exceed 10 MHz and even the older ones can run in the MHz range. Any reason not to use a longer USB cable or hub and keep the programming cable fairly short? \$\endgroup\$– PeterJCommented Feb 4, 2015 at 10:39
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\$\begingroup\$ 2' should be fine -- but I wouldn't go any longer than that (YMMV) -- if you need to go longer, I would do what @PeterJ suggests and get a longer USB cable instead, since those can go about 10' to 15' before you need an active repeater/hub. \$\endgroup\$– BrainSlugs83Commented Mar 27, 2015 at 17:10
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There is no formal maximum length cable (within sensible reason) and the maximum reliable length varies with the quality of the implementation.
As the cable gets longer the capacitance and inter conductor noise coupling rises* and depending on design decisions (or lack of decisions) by the makers, maximum reliable length will vary.
There are some designs which may be "flaky" at 1" cable length and I'd start to get nervous using an eg 10 foot cable with even the best of designs (but it MAY work OK). But, 1 or 2 feet should be OK in most cases. Erring on the shortest sensible length will do you no harm electrically.
Trying 2 feet and seeing what happens costs you an extra termination at one end (or a reused plug if you are clever and lucky) if you have to shorten the cable due to reliability issues.
*Inductance and resistance increase as well but I'd generally expect capacitive coupling and loading to be the key issue.
answered Feb 3, 2015 at 22:25
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1\$\begingroup\$ I wonder what sensible reason could mean? \$\endgroup\$– OctopusCommented Feb 4, 2015 at 4:43
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\$\begingroup\$ @Octopus I couldn't sensibly say :-). But I'd guess that the phrase "within reason" may have been felt to be a bit too commonplace" and not noticed as a genuine suggestion that things not be overdone, and/but that 'sensible reason' was perceived in the eye of the beholder and enscriber as standing out usefully more. Gargoyl's NGRam viwer suggests that ... \$\endgroup\$– Russell McMahon ♦Commented Feb 5, 2015 at 0:12
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\$\begingroup\$ ... **NGRams here ** "sensible reason' is several times more used than 'reasonably sensible' (especially before about 1885 :-) ) and slightly more used than "Arduino" except for the decade after Italy entered WW2, when "Arduino" took a real hit. | FWIW :-) \$\endgroup\$– Russell McMahon ♦Commented Feb 5, 2015 at 0:14