IBM B50 - Manual (Page 188)

S2-S3-S4 Definition for devices and FRUs not defined in the PCI specification.
PCI Base
Class
(lower
nibble) S2
PCI
Sub-Class
S3 -S4
Description
B
Processors
00 Reserved
01 Reserved
02 Reserved
10 Reserved
20 PowerPC
40 Co-processor
C
Serial bus controllers
00 FireWire (IEEE 1394)
01 ACCESS.bus
02 Serial Storage Architecture (SSA)
03 Universal Serial Bus (USB)
04 Fibre channel
Base Class S2 Sub-Class
S3 - S4 Description
0 10 AC Power
11 DC Power
20 Temperature-Related Problem
21 Fans
30-3x Cables
40-4x Terminators
50 Operator panels
60-6x Reserved
70-7x Reserved
90-9x Reserved
A0 ROS Heartbeat
B0 O/S Heartbeat
D0 Unknown device
E0 Security
A-4 7046 Model B50 Service Guide