B&B Electronics Parallel Port Input/Output Converter PPIO Benutzerhandbuch

14 

PPIO2899 Manual 

 
 

The parallel port’s status register stores the upper nibble (four 

bits) of the input line status. Since the smallest value that can be 
read from the port is a byte, some work must be done to get only the 
upper nibble’s value. We read a byte from the status register and 
bitwise AND it with F0h (240 decimal, 11110000 binary). This sets 
the lower nibble of the byte to zero. 
 
 

The parallel port’s control register stores the low nibble of the 

input line status. A method, similar to the one used to get the high 
nibble, extracts the low nibble. We read a byte from the control 
register and bitwise AND it with 0Fh (15 decimal, 00001111 binary). 
This sets the upper nibble of the byte to zero. 
 

 

 

Now, combine both bytes into one value by bitwise ORing them 

together. This value is stored in the variable Input_Byte. At this point, 
the upper nibble of Input_Byte is the same as the upper nibble of the 
status register and the lower nibble of Input_Byte is the same as the 
lower nibble of the control register. 
 

 

 

To return the status of a specific line, a test is done to determine 

the state of the line’s corresponding bit stored in Input_Byte. To 
determine the bit’s state, all other bits of Input_Byte are set to zero. 
This is done by bitwise ANDing Input_Byte with the mask value of 
the desired line shown in the following table. 
 

 

0 

01h 

1 

00000001 

1 

02h 

2 

00000010 

2 

04h 

4 

00000100 

3 

08h 

8 

00001000 

4 

10h 

16 

00010000 

5 

20h 

32 

00100000 

6 

40h 

64 

01000000 

7 

80h 

128 

10000000