Dwyer 16A - Manual (Page 18)

Page 18
Serial Communications
Options and Non-volitile 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.
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.
There is a third type of memory that is now currently used to combine the character-
istics of both RAM and ROM. This is known as EEPROM (electrically erasable pro-
grammable 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 pro-
grammed data even over long periods of time when the power is off. This is the type
of memory that all Love Controls uses to save the settings your program in your con-
trol. 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 begin oper-
ation.
If EEPROM is such a wonderful thing, you might ask, why bother with RAM? One
reason is that RAM is much faster than EEPROM. Faster speed gives you better per-
formance in critical control functions.
Perhaps the most important reason is that EEPROM has a limit to the number of
times 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 yo
ming
to us
Add
pictu
tions
cycle
Usua
pute
sitor
All 1
only
men
to R
upda
The
If yo
reco
mari
Mak
If yo
cont
949-1265:Layout 1 1/25/11 10:22 AM Page 18