Explanation of the reason to restrict the of DFB, TON, R_TRIG, F_TRIG, TOF and TP use in the first HSBY section.

Issue
Explanation of the reason to restrict the of DFB, TON, R_TRIG, F_TRIG, TOF and TP use in the first HSBY section.

Product Line
Quantum, M580

Environment
Unity, Control Expert

Resolution

Control Expert help shows the following restrictions for the M580 and Qunatum HSBY systems.




The following  provides an expalination of the reason for these restrictions:

The hot standby system allows, both on Quantum and M580, to execute some code on the standby PLC. This feature is meant to run some simple diagnostics on the Standby (or Wait) PLC which results can be sent back to the Primary PLC using the reverse registers (Quantum) or the HSBY DDT local and remote registers (M580).

Special care must be given to this code, since, by default, all the data from the primary is copied at the beginning of the cycle on the Standby PLC and therefore, the Standby PLC executes each cycle not on the data resulting from its previous cycle, but on those calculated from the last Primary PLC cycle. To avoid this unexpected behavior, both Quantum and M580 offer some control on what is executed on the Standby PLC and which data are not exchanged from the Primary to the Standby PLC.

Quantum:

But it is not possible to control the exchange of:

• Boolean à restriction on TRIGGER, TON … which are controlled by Boolean, and are using edges.
• internal data of DFB à restriction on DFB because, even if the external data are chosen with caution, the internal data are overwritten at each cycle from the Primary.
• (only) the first section is executed on the Standby PLC. the other sections are only executed in primary and (if configured by the user) in offline modes.

• Please note that the Offline mode is not concerned by these restrictions since no data is automatically exchanged from the Primary.
• It is possible to define an area of the state RAM which is not exchanged from Primary to the Standby:

M580:

• It is possible to choose to execute no section, only the first section or all the sections on the Standby PLC. This choice is common with the Wait mode.
• Please note that the Wait mode is not concerned by these restrictions since no data is automatically exchanged from the Primary.
• It is possible to choose, for any unlocated variable, if it is exchanged from Primary to Standby (by default, all variables are exchanged). No control is possible on located variables. For DFB, either all internal data of a Top level DFB and all nested DFB and FB are exchanged, either none.

Thus it is possible to run DFB or TRIGGER, TON, … on standby, taking care of which variables are exchanged or not. The customer must be aware also of the behavior in case of switchover or swap:

• A DFB executed on both Primary and Standby modes, and using only local variables (not exchanged) may start in Standby mode, using a standby context, and will continue its execution after a swap in Primary mode, but (at least during the first cycle) with the standby context. The same DFB will execute in Primary, then Wait, then Standby in case of SWAP command on the other PLC.
• A DFB executed on both Primary and Standby modes, and using only global variables (exchanged) will continue its execution with the Primary context (and therefore it is probably useless to execute it on Standby).
• Depending on the user choice about exchanged variables, many intermediate cases could occur.

It is why, as a rule of thumb, it is not recommended to use DFB, TRIGGER, TON, … and more generally any code that uses “state variables” (explicit or hidden) on a Standby PLC, and limit the code to combinatory evaluation, that are completed in one single cycle, only.

Note that these considerations are not specific to Control Expert, the same question exists in any Hot Standby system, with different responses depending on the Exchange data model used.

Released for:Schneider Electric Indonesia