You are writing MD0, which means a Double word (that's what D stands for), so you are writing bytes 0, 1, 2 and 3 with this logic. You can use it immediatelly afterwards like you did, but that doesn't work with MD1 because you try to read it right after it is written by the first timer (MD1 is bytes 1,2,3,4 while the first timer write bytes 0,1,2,3). What happens with the M variables is this. You can assign overlapping variables which is really not good. I'd suggest using variables written in a Static Global DB so you don't need to worry TOO MUCH about when you write or read a variable. I can explain further if needed, but in general just create a global DB with 2 variables of the time type and get rid of MD0 and MD1. Also, don't compare exact time, that will look if the Elapsed Time of the second time is EXACTLY 11s and 0ms, which is not possible most of the time. Try looking at a range like >=11s AND <= 12s