Advantech PCI-1751 User Manual page 30

BCD Select binary or BCD counting
BCD Type
0 Binary counting 16-bits
1 Binary coded decimal (BCD) counting
If you set the module for binary counting, the count can be any
number from 0 up to 65535. If you set it for BCD (Binary Coded
Decimal) counting, the count can be any number from 0 to 9999.
If you set both SC1 and SC0 bits to 1, the counter control register is in
read-back command mode. The control register data format then
becomes:
BASE+27 8254 control, read-back mode
Bit D7 D6 D5
Value 1 1 CNT
CNT = 0 Latch count of selected counter(s).
STA = 0 Latch status of selected counter(s).
C2, C1 & C0 Select counter for a read-back operation.
C2 = 1 select Counter 2
C1 = 1 select Counter 1
C0 = 1 select Counter 0
If you set both SC1 and SC0 to 1 and STA to 0, the register selected
by C2 to C0 contains a byte which shows the status of the counter.
The data format of the counter read/write register then becomes:
BASE+ 24/25/26 St at u s r e ad -b ack m o d e
Bit D7 D6 D5
Value OUT NC RW1
OUT Current state of counter output
NC Null count is 1 when the last count written to the counter
register has been loaded into the counting element
26 PCI-1751 User's Manual
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D4 D3 D2 D1 D0
STA C2 C1 C0 X
D4 D3 D2 D1 D0
RW0 M2 M1 M0 BCD