IBM Personal System/2 50 - Presence Detect Encoding

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 Module Presence Detect Signals
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
Modei 50 System Board — October 1990 3-11