The power modules that were installed on the chassis when the standby MM was once primary. The time of the last update can be obtained by querying rbistUpdateTimePrimary object. The power modules are listed as a bitmap. The bitmap is a string containing a list of 1's and 0's that is as long as the number of power modules supported. If a power module was installed in a bay, a 1 is present in the string in the position representing the bay ID as read from left to right. For example, if power modules were installed in bays 1 and 3, and 4 power modules were supported, the string would look like '1010'.