6947ch02.fm Draft Document for Review April 7, 2004 6:15 pm
26 IBM eServer zSeries 990 Technical Guide
2. The Motor Scroll Assembly (MSA)
3. The Motor Drive Assembly (MDA)
– MDAs are found throughout the frames to provide air cooling where required. They are
located at the bottom front of each cage, and in between the CEC cage and I/O cage 1
in combination with the MSAs.

Hybrid cooling system

The z990 has a hybrid cooling system that is a breakthrough in lowering power consumption.
Normal cooling is provided by one or two MRUs connected to the heat sinks of all MCMs in all
books.
If one of the MRUs fails, backup MSAs are switched in to compensate for the lost refrigeration
capability with additional air cooling. At the same time, the oscillator card will be set to a
slower cycle time, slowing the system down by up to 8% of its maximum capacity, to allow the
degraded cooling capacity to maintain the proper temperature range. Running at a slower
cycle time, the MCMs will produce less heat. The slowdown process is done in steps, based
on the temperature in the books.
Figure 2-3 shows the refrigeration scope of MRU0 and MRU1.
Figure 2-3 MRU scope
2.1.2 Models
The z990 has four orderable models. The model numbers are directly related to the number of
books in the system and the maximum number of PUs that can be characterized by the
installation. For customer use, PUs can be characterized as CPs, IFLs, ICFs, zAAPs, or if
need be, additional SAPs.
򐂰The IBM 2084 model A08 has one book (A) with 12 PUs, of which eight can be
characterized by the customer. The four remaining PUs are two system assist processors
(SAPs) and two spares.
򐂰The IBM 2084 model B16 has two books (B) with 12 PUs in each book for a total of 24
PUs, of which 16 can be characterized by the customer. The four remaining PUs are four
system assist processors (SAPs) and four spares, two of each in each book.
򐂰The IBM 2084 model C24 has three books (C) with 12 PUs in each book for a total of 36
PUs, of which 24 can be characterized by the customer. The remaining PUs are six
system assist processors (SAPs) and six spares, two of each in each book.
Book 0Book 3 Book 1 Book 2
MRU 0 MRU 1