3-7. Presence Detect Encoding - IBM Personal System/2 50 Technical Reference

The 'presence detect' signals are used by the system to determine
memory card size and memory speed. The pins are either connected
to ground (G) or not connected (N). The following table shows those
combinations supported by Model 50.
Type of Memory Presence Detect Slgnall
Module Package 0
1
2 3
1 MB Memory at 120 ns G N N N
1 MB Memory at 85 ns G N N G
2 MB Memory at 85 ns N
G
N G
Figure 3-7. Presence Detect Encoding
Model 50 System Board 3-11