Understanding The Parallel Port

On the PC the parallel port uses a 25-pin connector (called DB-25, 25-pin D-sub or 25-pin D-shell), as you can see on Figure 2. On printers, however, a different kind of connector is used, called Centronics, which has 36 pins.

click to enlarge
Figure 2: The parallel port.

Besides the eight data bits, there are more signals available on the parallel port. On the table below we list all the basic parallel port signals and their function, as well as their location on both standard 25-pin and Centronics connector. The I/O column indicates if the signal is input (I) or output (O). Input means that the signal must come from the device to the parallel port (i.e. the signal must be provided by your prototype); output means that the signal comes from the parallel port.

Signal	Name	Pin (25-pin standard connector)	Pin (36-pin Centronics Connector)	I/O	Description
/STROBE	Strobe	01	01	O	Indicates if data is ready or not to be transmitted.(0 = Data ready to be transmitted, 1 = Data not ready to be transmitted)
/ACK	Acknowledge	10	10	I	Indicates that the printer is ready to receive data.
BUSY	Busy	11	11	I	Indicates that the printer is not ready to receive data.
PE	Paper Empty	12	12	I	Indicates that the printer has no paper to print on.
SELECT	Select	13	13	I	Indicates that the printer is on its “on line” state, ready to get information.
/AUTO FD XT	Auto Feed	14	14	O	The printer moves the paper to the beginning of the next line.
/ERROR	Error	15	32	I	Some error occured (printer disabled, paper empty).
/INIT	Init	16	31	O	Resets the printer and clears its printing buffer.
/SELECT INPUT	Select Input	17	36	O	Data can only be transferred to the printer when this line is set to “0”.
D0 thru D7	D0 thru D7	2 thru 9	2 thru 9	O	Data bits.
GND	Ground	18 thru 25	19 thru 30	O	Ground.

The parallel port uses three I/O addresses: data (378h), status (379h) and control (37Ah). If you want to send data to the parallel port and get this data outside the computer, just write this data to the parallel port data address. For example, if we want to turn on all our LEDs, all we need to do is to send the value 255 (which is the decimal equivalent for 11111111, i.e. all data bits set to “on”) on the address 378h. Of course we will explain more about this and also the role of the status and control addresses.

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