1. The centronics port
Interfacing to a PC

2. Connection
Devices are connected to the parallel port (LPT) using a 25-way D-connector.

3. Pinout of centronics port

4. Outputs We use D0 to D7 for most outputs
D7 is the most significant bit
We output all eight bits at once
To get this output send (binary), 156 (decimal) or 9C (hex) to the output port D7 D6 D5 D4 D3 D2 D1 D0 1

5. Address The output port is at address 378H Outputs use the command:
OUT address, data
Hexadecimal numbers can be written using &h at the start
You can send 9CH to the output port using:
OUT &h378, &h9C

6. Inputs The inputs are at address 379H
The inputs can be read using INP(address)
INP(&h379) behaves like a variable and has a value that depends on the inputs so is used in statements like this:
S=INP(&h379)

7. Addresses of pins

8. Inputting S is now equal to 9FH
To look at the input from the ERROR pin we need to input from the input port
S=INP(&h379) D7 D6 D5 D4 D3 D2 D1 D0
BUSY
ACK PE
SLCT
ERROR
Not used 1
S is now equal to 9FH

9. Masking
All the bit other than D3 need to be ignored – this is called masking D7 D6 D5 D4 D3 D2 D1 D0
BUSY
ACK PE
SLCT
ERROR
Not used 1

10. Making the mask The function AND is used for masking
Put 0s where you want the bits to be ignored
If D3 is 0 the answer is zero
If D3 is 1 the answer is not zero D7 D6 D5 D4 D3 D2 D1 D0
Hex
BUSY
ACK PE
SLCT
ERROR
Not used
AND 1 9F 08
Answer

11. Code for inputting and masking
S=INP(&h379)
S=S AND &h08
IF S=0 THEN …
IF S>0 THEN …