I'm very new to PLCs and trying to implement a program to have a pneumatic cylinder cycle between 2 positions every few seconds. I'd like the frequency of that delay  to be user selectable on an HMI when the cylinder is not operating. I'm using an S7-1200 with a KTP400 on TIA portal V17. 

I've set the PV of the TON block to a "Time" tag. I've linked an I/O on the HMI to this tag. However, whenever I interact with the I/O, if I set a value of "255" it stays once I exit the keypad, but if I enter "256" the value resets to "0". If I enter "257" the value resets to "1" and so on. TON seems to operate off of the value that it resets to, I.E. if I enter "257" and it resets to "1" the time is much faster than 255. 

I'm sure it's some thing stupid but I don't understand what's going on. The HMI I/O is set to display in decimals with 6 digits. The HMI tag is linked to the PLC tag.... what am I missing? 

I've solved it.... by stabbing in the dark and I don't know what it means. 

I tried deleting the HMI tag, re-creating the HMI I/O, etc... and nothing worked. 

I saw 1 thread that mentioned when you create PLC tags you should let TIA specify the memory address for you and only create the name yourself (as opposed to manually specifying the mem. address and then renaming). I created a new DINT tag with a new name and lo and behold it works. I manually changed the address to %MD4 and it went back to the old bug, then manually renamed back to %MD7 and it works again. 

Obviously I don't understand something here.... what is it? Can I never change the memory address even though nothing else is using that?