EEPROM Memory Failures
On West brand products, all non-volatile parameters are stored in EEPROM so that they are remembered after power down.
EEPROM's have a limited of data “writes” before they start to fail. The EEPROMs used in West N & P-series products allow >100,000 write cycles. This is a large number but would not be enough if a parameter such as the setpoint was continuously written to it every few seconds.
To avoid this problem, the last non-volatile parameter changed is only stored in normal RAM which isn’t limited in this way.
If a new value for the same parameter (a different value for the setpoint for example) is detected, the new value is again written only to normal RAM, and nothing is written to EEPROM.
This last changed value (known as the "Pending Parameter") is only written to EEPROM under two conditions:
1. If the power supply circuit detects that the power is being turned off, it then instructs the CPU to transfer the "Pending Parameter" value into EEPROM. The design of the power supply circuit ensures that there is sufficient time from power-off to write this value before it shuts down completely.
2. If a different parameter is changed (perhaps the alarm value is changed).
In most single loop controllers, there is only one Pending Parameter, so the new alarm value would be stored in normal RAM and the previous Pending Parameter would be written into EEPROM.
The above strategy works well for normal operations, but users should not constantly write to two different parameters via comms as this would use up the 100,000 writes quickly. Normally only the setpoint needs to be updated frequently, and other parameters only change rarely, but customers using serial communications to change parameters must ensure that their master device doesn’t write to constantly to more than one parameter. They should only make other changes when necessary in order to protect the EEPROM.
