May, 2013
Page 22 of 52
949-1194 Rev. 7
SERIAL COMMUNICATIONS OPTIONS AND NONVOLATILE MEMORY
There are many different types of memory used in computer driven devices. The terms
RAM (random access memory) and ROM (read only memory) are a couple with which you
may be familiar.
RAM is used in computers to run programs and hold data for a short period of time. This is
the memory that is used primarily in PCs. RAM is very fast and can be read and written to
over and over again. Its major weakness is that it is erased when the power is turned off.
ROM is used in computers to hold the ‘permanent’ programming that allows a PC to start.
This memory is ‘burned in’ to the chip itself and can not be changed. Unlike RAM,
however, this memory is permanent. While it can not be changed, it can not lose its
programming when power is turned off. This is the type of memory that is used to store the
permanent programming for the control.
There is a third type of memory that is now currently used to combine the characteristics
of both RAM and ROM. This is known as EEPROM (electrically erasable programmable
read only memory). While the name may be long and somewhat cryptic, the EEPROM can
be erased and re-written many times, and yet hold the programmed data even over long
periods when the power is off. This is the type of memory that Love Controls uses to save
the settings you program in your control. The reliability and longevity of the data retention
is what allows us to guarantee a 10 year data retention without power.
In normal operation, the control uses RAM, just as any other computerized device.
Whenever you make a change to one of the parameters in the control, the set point for
example, the new value is written into the EEPROM. This way, if power goes off for
whatever reason, when power resumes, the latest settings are preserved. When power is
turned on, the data is copied from the EEPROM to the RAM to restore operation.
You might ask, “If EEPROM is such a wonderful thing, why bother with RAM?” One reason
is that is that RAM is much faster than EEPROM. Faster speed gives you better
performance in critical control functions.
Perhaps the most important reason is that RAM allows an unlimited number of writes,
while EEPROM has a limit to the number of times that it can be erased and re-written.
Current technology now sets that limit at about one million erase / write cycles. In a
dynamic control situation, it may be necessary to update RAM every few milliseconds.
EEPROM can not keep up to that pace, and, even if it could, it would be ‘used up’ in a
matter of days.
If you think about how long it would take to make a million changes to the control
programming through the front key pad, you will see that it would take a very long time to
get to use up the life of the EEPROM.
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