The "maintenance free" MMC card, and what Copy-RAM-to-ROM is really good for.
I think others might benefit from what I have just learned.
The scenario is this:
I use a machine with an S7-300 CPU as my device for creating modified programs to send to end-customers, in the shape of an MMC card they just have to put into their own similar machine.
I test the program together with the HMI and i/o.
According to Siemens, when you shut down the CPU it will write the latest status of all the data and code blocks to the MMC card. It does so with the energy stored in its internal power supply circuitry.
The problem is, that it takes a longer time than one thinks (30 seconds ?), and if you remove the card too soon, the saving on the card fails. In that case, all the DB's gets "initialised" which in turn may be a small catastrophe.
The problem is aggrevated by that there is no indication on the CPU that it is writing to the MMC card, so you have no idea of when you can remove the card.
If you use the Copy-RAM-to-ROM function before shutting down the CPU, then it seems to keep the current actual values in the DBs after a shutdown and removal+reinsertion of the MMC card.
I think this is very dependant on how many DBs there is in the program. I shall try to make some experiments to get to the bottom of this.
edit: This probably also explains the limitation of how much memory may be allocated as retentive for each CPU type in question. It depends on how much juice there is in the power supply.
But my conclusion is this:
If you want to remove the MMC card, in order to use the program on the card in another similar PLC and machine, then use the Copy-RAM-to-ROM function before shutting down the PLC !
I think others might benefit from what I have just learned.
The scenario is this:
I use a machine with an S7-300 CPU as my device for creating modified programs to send to end-customers, in the shape of an MMC card they just have to put into their own similar machine.
I test the program together with the HMI and i/o.
According to Siemens, when you shut down the CPU it will write the latest status of all the data and code blocks to the MMC card. It does so with the energy stored in its internal power supply circuitry.
The problem is, that it takes a longer time than one thinks (30 seconds ?), and if you remove the card too soon, the saving on the card fails. In that case, all the DB's gets "initialised" which in turn may be a small catastrophe.
The problem is aggrevated by that there is no indication on the CPU that it is writing to the MMC card, so you have no idea of when you can remove the card.
If you use the Copy-RAM-to-ROM function before shutting down the CPU, then it seems to keep the current actual values in the DBs after a shutdown and removal+reinsertion of the MMC card.
I think this is very dependant on how many DBs there is in the program. I shall try to make some experiments to get to the bottom of this.
edit: This probably also explains the limitation of how much memory may be allocated as retentive for each CPU type in question. It depends on how much juice there is in the power supply.
But my conclusion is this:
If you want to remove the MMC card, in order to use the program on the card in another similar PLC and machine, then use the Copy-RAM-to-ROM function before shutting down the PLC !
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