More of what dmroeder has stated....
Just about every PLC has 'special bits' these bits are preconfigured in the CPU, automation direct (Koyo) has SP bits, SP4 and SP5 for example are bits that turn on and off at SP4 true and false every second, SP5 100Millisecs
Allen Bradley has S2:4/4 and S2:4/5, Mitsubishi has SM412 and SM410, S7 200's SM0.5 etc...