Why does a PLC have to use a backup battery? Isn’t ROM a good alternative?
Thread Content
Why does a PLC have to use a backup battery? Isn’t ROM a good alternative? ROM is read-only memory; it ensures that data is not lost when power is cut off, and it is generally used to store important data. RAM is random-access memory, and its data is lost when power is disconnected. At the very least, the program storage should be designed as ROM, while the data storage should be designed as RAM. Why then are more and more PLCs from various brands using backup batteries?
The access speed of ROM is slower than that of RAM; RAM allows for faster data updates, making it suitable for processing dynamic data. I’m not sure if what I said is correct
When the system loses power, the battery can ensure that the programs in RAM continue to run. With ROM, initialization is required after power is restored; many data are lost, the status on-site becomes inconsistent, and there are more sudden problems
ROM has its own functions, such as storing system programs and pre-written applications. The backup battery also has its own purpose: it supplies power to the system in the event of a power outage, thereby preventing the loss of intermediate results that are being processed in RAM. These are merely my humble opinions. :)
There are PLCs that don’t require a backup battery; our factory has two such units in use!
Go back upstairs; generally, there are devices that can perform similar functions even without batteries, to ensure that data is not lost, such as capacitors.
Some PLCs now use TF flash cards to ensure speed and data integrity.
Bro, can the battery of that ROM last that long?
ROM is read-only memory that retains data without power
PLCs can also function without batteries; generally, such PLCs use flash memory to store data, and the data is not lost when power is lost since it is stored permanently in the flash memory.
After all this discussion, the original poster should have some understanding by now. It depends on where the product comes from, as well as the characteristics of DCS or PLC. Siemens seems not to have batteries, while GE does; let’s discuss this further.
Yes, as mentioned above, Siemens stopped using batteries years ago; the CPUs of the S7 series must have a memory card in order to function. The actual data in operation is stored on the memory card; it can be saved in the event of a power outage, while resetting the CPU will erase this actual data.
The user programs of PLCs need to be applied in practice and modified; using ROM is clearly not suitable, so E2PROM is used instead. Even if the values are fixed, there are still many system configuration parameters such as timing settings and counting constants. With a backup battery, these constants remain intact after a power outage, and they can be used once power is restored.
ROM cannot be restored to its operating state after a power loss; parameters such as timers will revert to their initial values. RAM combined with a battery can prevent this problem. Small PLCs generally use ROM; for example, the Siemens 300 uses MMC cards, while the Siemens 400 typically relies on RAM combined with a battery
That’s correct; those applications that have high requirements for process data generally use batteries or capacitors, and they are often found in large PLCs. At the same time, large PLCs are also equipped with ROM. On the one hand, after a power loss and restart, the CPU directly uses the intermediate data stored in RAM and the programs that are currently running ； On the other hand, after restarting due to a prolonged power loss (damage to the battery or capacitors), the CPU loads the programs from ROM into the RAM of the PLC.