Thoughts on Trailing Curtain Sync

Joseph McAllister pentaxian at
Sun Jul 4 16:20:54 EDT 2010

On Jul 2, 2010, at 13:53 , John Sessoms wrote:

> The K-10D and K-20D both allow you to set the on-board flash to  
> trailing curtain sync. How difficult would it be for them to make  
> the hot-shoe also fire on the trailing curtain when a non-dedicated  
> "flash" is installed?
> E.G. like with a radio-sync transmitter or Vivitar 285HV? Dedicated  
> flash like the AF-540FGZ has its own circuitry to talk to the camera  
> and can be set to trailing curtain sync.
> Is this something they could do with a firmware update?
> Or is it something that would require radical hardware revision?
> For that matter what do the dedicated pins on the Pentax shoe tell  
> the flash? I know what the big one in the center does, and it's  
> obvious which one is ground.

Everything you need to know except the actual format and content of  
the "digital" signal and the "mode" format.

Here's a diagram of a circuit to isolate an old high voltage flash  
from the hot shoe about half way down the page. I know, you didn't  
ask, but, it can't hurt to have a copy on your hard drive!

And the Hot Shoe and 5P cable pins, in case you wanted to know. Near  
bottom of page.

Lots and lots of data about the Pentax AFxxxFGZ flashes compared to  
others manufacturers.

also a more technical tome:

Joseph McAllister
pentaxian at

Grant me the senility to forget the people
I never liked anyway,
The good fortune to run into the ones I do, and
The eyesight to tell the difference.

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