Terry
The reason for multiple timer types in Siemens S7 goes back to the days when everybody, including the beloved TI, had their own way of controlling time. Most maufacturers had their own design on what a timer should look like, how it should behave, what the limits of acceptable values were, what timebases were available etc. Siemens, in their venerable S5 range, had several timer functions all based around a common way of holding a time value in a 16-bit register. This single variable held both the timebase and the magnitude of the time value. The S7 range then inherited this timer type as a form of backward compatibility with S5 structures.
I appreciate that to all good ol' boys on your side of the pond the IEC must seem like a pinko commie conspiracy led by those cheese-eating surrender monkeys in Europe, but I still believe there are elements of value in the 61131-3 software document. One of the most valuable of these is the idea of absolutely standard data types across all manufacturers. This includes a variable type called TIME. The next obvious extension of this is standard functions for manipulating these data types. So the IEC standard defines three simple timers: an on-delay, an off-delay and a pulse generator. S7 includes these standard IEC timers as well.
Finally, Siemens even have an instruction library entitled TI-S7 Conversion Library which includes as many of your favourite TI operations as you could hope for. Function FC80 is basically the TI timer function coded for S7.
So with S7 you've got the possibility of using S5 timers, if that's where you've come from; TI timers, if that's where you've come from; or IEC timers if you want to make your code transportable. I know some people get very confused with all this (which one should I use?) but it's a bit like complaining a dictionary has too many words: nobody says you have to use them all, just take the ones you need.
Regards
Ken.