Which type of Memory card should use so that program remains stored if power fails?

Hello shek;

You can use as Load memory to a S7-400 either a RAM or a flash EPROM memory card.

A RAM card requires battery backup, but allows downloading while the CPU is in Run mode, using the Download command.

A flash card retains the program without battery backup, but the CPU must be in Stop mode when you do the program download, using the Load User Program to Memory Card instruction.

In our most complex projects with S7- 400 CPUs we would use a RAM card during commissioning and testing, because we would need to modify and download the program often, and doing so in Run mode was easier. When the project was transferred to he client, we would change the memory card to flash so the downloaded program would be maintained without battery support, and modifying the program would require a bit more knowledge for the user (and the user generally does not like to put the CPU in Stop mode during production).

The following document will give you much more information on the subject:

How can I get an overview of the S7- 400 memory concept?

One of the links presented at the end of the documents will bring you to a list of available memory cards for the S7- 400, both flash and RAM.

Hope this helps, 

Daniel Chartier