PLC Scan, Flags & Registers

[sid 0]

Once I have asked question in forum about the flag updation in programme. There was reply that flags such as M0 are updated as soon as logic processing of rung. Is the same is true for data registers i.e when we use -[mov k10 D0]- instruction, D0 is updated as soon as rung processing or it is updated at the end of scan.

[The Cat 0]

Updating of devices depends on the mode of operation of the PLC and the model of PLC being used. With some versions there is no choice of operating mode, with others it is selectable. Generally, in (batch) refresh mode, M coils and register devices are updated in "real time" whereas X inputs are updated at start of scan and Y outputs, timers and counters are updated at end of scan. In direct mode, Xs and Ys are updated by reference to the device state as it is addressed by the program i.e. in real time. Other devices are as for refresh mode. Both modes have advantages and disadvantages depending on the application!

[funfrok 0]

Is there a mode that, M and D devices aren't updated real time? I think it is impossible. Also "timers and counters are updated at end of scan." doesn't make sense to me. How do you configure these modes?

[The Cat 0]

To my understanding, the update of M and D devices cannot be changed. Certainly, the older FX range of PLCs operated only on refresh principal - I believe that subsequent models do the same. Again, older A and AnS series products (possibly not all) had a switch in the CPU that determined the operating mode. It is my belief that Q series utilises the refresh mode also but benefits from the additional refining program options such as SCAN, INITIAL, SLOW, CONSTANT. The FX programming manual or one of the A series user /fundamental manuals should explain more clearly how C and T devices work and give examples of the errors in accuracy that can occur as a result of dependancy on scan update. Updating of C and T devices refers to the setting of the "completed" coil -not the running value Edited 11 Oct 2006 by The Cat