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<< Click to Display Table of Contents >> General explanation of over-booking |
  
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<< Click to Display Table of Contents >> General explanation of over-booking |
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When a help symbol is booked against a mirror symbol, connection numbers from the mirror are transferred to the help symbol, and cross-references are updated both in the help symbol and in the mirror. The connection numbers that are updated in the help symbol, define the booking. In other words, they define which function in the mirror (contact in contact mirrors) that the help symbol is assigned to.
The booking normally takes place when a help symbol (for instance a help contact) is inserted and assigned an item designation that corresponds to a mirror symbol.
However, booking will also take place the other way around, meaning if the mirror symbol is inserted after the help symbols. In that case, the situation is however somewhat different. If no mirror symbol exists, it is possible to enter connection numbers manually. If a contact mirror do exist, you can only enter contact numbers that are available in the contact mirror, either by typing them or by selecting them. If you try to enter contact numbers that do not exist in the mirror, they will automatically be replaced with correct numbers from the mirror, and if no contact is available, the contact numbers will be replaced with asterisks/stars ("*"). This is all normally fine.
However, when replacing a contact mirror, this may in some situations have undesired consequences. Please consider the following imaginary scenario:
You have a contact mirror and a few contacts that are booked against that contact mirror. You find out that the contactor you have selected has an insufficient number of contacts. You therefore need to replace the contact mirror. You therefore delete the original contact mirror. When doing so, the contacts will keep their contact numbers, despite the fact that the contact mirror that they received those contact numbers from, is now gone. If you would insert the same contact mirror again, a new booking would take place. When doing that, contacts will first book against free contacts with identical numbers. Only if that fails, a booking against the entire mirror will take place, which in turn might lead to changed connection numbers in the help symbol. The consequence, when inserting the same contact mirror again, is that the original booking will be regained. When you now instead insert a different contact mirror, booking will be made against that. Suppose that you choose the wrong one, perhaps one with even fewer contacts. Most of your existing contacts would then fail to book. That will normally be shown in the circuit diagram with connection numbers that are replaced with asterisks ("*"). When that happens, when the original connection numbers are replaced with asterisks, information about the original booking is lost. If you switch back to the original contact mirror, there is no information left about which contacts where booked against which positions in the contact mirror. A brand new booking will take place, which may differ from the original one. Contacts may therefore be swapped in an uncontrolled manner.
At a cost that will be discussed below, you can use the project parameter in question here, to reduce this problem. If you de-activate the Replace connection number in help symbol with * parameter, the original connection numbers will not be replaced with asterisks when matches towards the contact mirror disappears, like if the contact mirror is replaced as described above. When a better suited contact mirror finally gets in place, contacts will find their positions in that with priority for contacts with the same connection numbers that the contact originally had. This is a way of preserving information and to reduce the risk of unintentional rebooking. However, it comes at a price. A situation with an over-booked contact mirror, meaning that you have inserted more contacts than is physically available, will not be as obvious as it would otherwise be. Contacts with non-existing contact numbers with correct cross-references to the main symbol, will not give a clear indication of the problem that does exist. Using the standard setting with Replace connection number in help symbol with * activated, the contact numbers of such a contact would be replaced with stars, which would signal the error in a clearer way.
The bottom line is that both ways have their advantages and dis-advantages. You will have to make the compromise that suits your needs best.
Please refer to the two sub-topics below, which contains examples that illustrate the consequences of the two alternatives that are discussed above.
Replace connection number in help symbol with *: