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 you think about how long it would take a million changes to the control program-
ming 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.
Adding one of the computer communications options (e.g. C4, LV) changes the pic-
ture. The speed of computer communications is such that hundreds of instructions
can be made in less than a minute. In such a situation, the million erase/write cycles
could be used up in a couple of months causing the control to fail.
Usually in such a situation, the control is under close observation by the host com-
puter. It may not be necessary, then to the data written to the EEPROM, as it is ‘tran-
sitory’ in nature (changing set points for a ramp/soak sequence for example).
All CN74000 Series controls with communications options made before April 2001
are only able to write to the EEPROM. Controls manufactured after this date have a
menu item in the Secure menu (Stor) that allows the serial communications to write
to RAM (Stor = no) with a special write command that allows to EEPROM to be
updated or written directly to EEPROM (protocol command 0442).
The factory default is ‘write to EEPROM’ (Stor = YES).
If your computer system will be making frequent changes to the control we strongly
recommend that you select the ‘write to RAM’ parameters (Stor = no). If you are pri-
marily reading from the control, there is no need to change the setting.
Any instruments equipped with any Serial Communications are
limited to one million WRITE cycles to the EEPROM through the
Serial Communications Port. Exceeding this limit will generate
a FAIL TEST error. There is no limit to the number of times you can
READ from this instrument EEPROM.
Make sure that the software you use does not write too often to the instrument.
If you have any questions regarding how your software works with the instrument(s),
contact your System Administrator, Programmer, or Software Supplier.