Specifications

Port B

Port B (I/O address 61h) supports the following settings. Bits 0 to 3 are read/write,

bits 4 to 7 are read-only

5.5

Port 61h Data Port Description

KEYB PRNMI INOMI SP SF IOCHK PR SPON SPCOU

r r r r r/w r/w r/w r/w

D7 D6 D5 D4 D3 D2 D1 D0

Bit Name Function

7 PRNMI parity error occurred

0 = no NMI from parity error

1 = parity active, NMI requested

6 IONMI I/O error occurrend

0 = no NMI from IOCHK

1 = IOCHK active, NMI requested

5 SP Loudspesker counter signal

4 RF Refresh detection,changing with the refresh cycle (15 µs)

3 IOCHK I/O channel check

0 = enabled

1 = disabled and erased

2 PR Parity checking on systom board

0 = enabled

1 = disabled and erased

1 SPON Direct speaker control

0 = speaker off

1 = speaker on

0 SPCOU Loudspeaker counter gate

0 = counter disabled

1 = counter disabled