Originally posted by kaseki I'm not sure what you mean by digital protocol for TTL
It is secret knowledge! Starting in 1980, Pentax began use of a four-contact (center, ready, mode, and ground/rail) hot shoe that debuted on the LX and continued on the A-series, P-series, and 645 cameras. Initial TTL support was provided using this shoe on the LX, Super Program, and 645 using so-called analog dedication. The body holds the "mode" contact low until sufficient light has been detected at the film plan, signaling "quench" to the flash by setting it high. In 1987, a second (digital) dedication/TTL protocol was added to the line using a five-contact (center, ready, mode, data and ground/rail). This second protocol allows for purely digital flash dedication and TTL flash with the five contact flash continuing to this day. As hinted by the name, the digital protocol used the data pin with the other contacts used for legacy support.*
This is where things get interesting in that there were soon three different levels of support from flash (analog only, digital only, and either) as well as three levels of support from bodies. The analog protocol along with supporting flash had a very long life courtesy of the long-lived LX camera and analog-only flash were similarly long-lived. I don't know for sure when the AF400T, AF280T, and other analog models left the shelves, but I do know that body support continued through to the MZ-S (introduced 2001). Flash support for analog protocol bodies continued through to AF360FGZ and AF540FGZ. Yes, the AF540FGZ will work quite seamlessly doing TTL on my 1983 Super Program. I believe that the AF200FG is the last model that supports digital TTL.
Well, there you have it...TMI? Today we have only P-TTL, but strangely enough our dSLR bodies retain remnants of analog protocol dedication support. My AF280T will quite happily tell the K-3 it is there and what aperture to use for auto-thyristor flash with the currently set ISO...even auto-ISO.
Steve
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EDIT: This is purely an assumption on my part and not based on testing with digital TTL gear.