I know if I use these two directly with each other, I will NOT get TTL, but if I use a TTL cord to connect them, will I cause irreparable harm to either or both? Or even reparable (but expensive) harm? I have a PC-to-PC sync cord I can use if required. The reason for this inquiry in the first place is that I have an external battery pack that will give me 500 full-power flash pops on a single charge, but it only has power cords to connect to the Canon flash. I have a Contax TLA 280 flash but it's pretty wimpy in comparison to the 580EX, or a Metz 45 CL-4, which I don't have SCA cords to use it TTL. I can always use the Metz with the Contax in automatic mode for now...