Parallel port modes
The IEEE 1284 Standard which has been published in 1994 defines five modes of data transfer for parallel port. They are,
1) Compatibility Mode
2) Nibble Mode
3) Byte Mode
4) EPP
5) ECP
Hardware
The pin outs of DB25 connector is shown in the picture below
Parallel Port
The lines in DB25 connector are divided in to three groups, they are
1) Data lines (data bus)
2) Control lines
3) Status lines
As the name refers , data is transferred over data lines , Control lines are used to control the peripheral and of course , the peripheral returns status signals back computer through Status lines. These lines are connected to Data, Control And Status registers internally . The details of parallel port signal lines are given below
Pin No (DB25) Signal name Direction Register – bit Inverted
1 nStrobe Out Control-0 Yes
2 Data0 In/Out Data-0 No
3 Data1 In/Out Data-1 No
4 Data2 In/Out Data-2 No
5 Data3 In/Out Data-3 No
6 Data4 In/Out Data-4 No
7 Data5 In/Out Data-5 No
8 Data6 In/Out Data-6 No
9 Data7 In/Out Data-7 No
10 nAck In Status-6 No
11 Busy In Status-7 Yes
12 Paper-Out In Status-5 No
13 Select In Status-4 No
14 Linefeed Out Control-1 Yes
15 nError In Status-3 No
16 nInitialize Out Control-2 No
17 nSelect-Printer OutControl-3 Yes
18-25 Ground - - -
Parallel port registers
As you know, the Data, Control and status lines are connected to there corresponding registers inside the computer. So by manipulating these registers in program , one can easily read or write to parallel port with programming languages like ‘C’ and BASIC.
The registers found in standard parallel port are ,
1) data register
2) Status register
3) Control register
As there names specifies, Data register is connected to Data lines, Control register is connected to control lines and Status register is connected to Status lines. (Here the word connection does not mean that there is some physical connection between data/control/status lines. The registers are virtually connected to the corresponding lines.). So what ever you write to these registers , will appear in corresponding lines as voltages, Of course, you can measure it with a multimeter. And What ever you give to Parallel port as voltages can be read from these registers(with some restrictions). For example , if we write ’1′ to Data register , the line Data0 will be driven to +5v. Just like this ,we can programmatically turn on and off any of the data lines and Control lines.
