Tandy 102 Service Manual page 98

(1) Programming the Command/Status (C/S) Register
The contents of the command register can be written during an I/O cycle by addressing it with an
I/O address of xxxxx000. Bit assignments for the register are shown below:
TM1 TM2 IEB IEA PC2 PC1 PB PA
Timer command
Definition of PAo - 7 0 = input
Definition of PBo -,. 7
1 = output
00 = ALT1
See the port
11 =ALT2
control
Definition of PCo - 5
01 =ALT3 assignment
10 =ALT4 table.
Port A interrupt enable
1 = enabled
Po
rt
B interrupt enable 0 = disabled
00 = NOP : Does not affect counter operations.
01 = STOP : Stops the timer if it is running.
NOP if the timer is not running.
10 = STOP AFTER TC : Stops the timer when it reaches
TC.
NOP if the timer is not running.
11 = START: If the timer is not running, loads the mode
and the count length, and immediately
starts timer operation. If the timer is run-
ning, loads a new mode and the count
length, and starts timer operation imme-
diately after TC is reached.
Figure C-8. Programming the Command/ Status Register
Pin
ALT1
ALT2 ALT3 ALT4
PCo
Input po
rt
Output port A INTR A INTR
PCB Input port
Output po
rt
A BF A BF
PC2 Input po
rt
Output po
rt
A STB
A STB
PC3
Input po
rt
Output port
Output port B INTR
PC4 Input po
rt
Output po
rt
Output po
rt
B BF
PC5
Input po
rt
Output po
rt
Output po
rt
B STB
Table C-5. Port Control Assignment