IBM Personal System/2 50 - System Control Port A (Hex 0092)

Bit Function
O4NWDA
AOA
Parity Check
Channel Check
Timer 2 Output
Toggle with Each Refresh Request
Enable Channel Check
Enable Parity Check
Speaker Data Enable
Timer 2 Gate to Speaker
Figure 3-24. System Control Port B (Read)
Bit7
Bite
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
Setting this bit to 1 during a write operation, resets IRQ 0.
When this port is read and this bit is set to 1, a parity
check has occurred.
When this port is read and this bit is set to 1, a channel
check has occurred.
When this port is read, this bit indicates the condition of
the timer/counter 2 ‘output’ signal.
When this port is being read, this bit toggles for each
refresh request.
Setting this bit to 0 enables channel check. It is set to 1
during a power-on reset.
Setting this bit to 0 enables parity checking. This bit is set
to 1 during a power-on reset.
Setting this bit to 1 enables speaker data. This bit is set to
0 during a power-on reset.
Setting this bit to 1 enables the timer 2 gate.
System Control Port A (Hex 0092)
Bit Function
7,6 Fixed Disk Activity Light
Reserved
Watchdog Timer Status
Security Lock Latch
Reserved
Alternate Gate A20
Alternate Hot Reset OA
NAAM
Figure 3-25. System Control Port A (Hex 0092)
Mode! 50 System Board — October 1990 3-25