Parallel Port

 

Parallel Port Description.
Parallel ports and interfacing.

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General description

 

Initially it was placed to the parallel port in the card of the “printer adapter of IBM PC”, or also with the card of the “monochrome monitor ”.

With the arrival from clones to the market, it is created a Multi IO Controller (Multi I/O) where settle disk controller, Serial ports, games port and the parallel port.

By now the parallel port is included commonly in the motherboard of the computer (MotherBoard). However, the connection of the port with the external world has not undergone modifications. This port uses a female connector DB25 in the computer and a special connector male Call Centronic  that has 36 pins.

It is possible to connect the DB25 of 25 pins to the Centronic of 36 pins since near half of the pins of the Centronic they go to ground and they are not connected with the DB25.

Description of DB25 connector  of the PC and DB25 connector  of the Centronic

Parallel port Conector DB25 male female

 

The parallel port is formed by 17 lines of signals and 8 gorund lines . The lines of signals are formed by three groups:

    • 4 Control lines

    • 5 Status lines

    • 8 Data lines

In the original design the control lines are used for the interface, control and interchange of messages from the PC to the printer.

The status lines are used for interchange of messages, indicators of state from the printer to the PC (lack paper, occupied printer, error in the printer).

The data lines only provide the data of impression of the PC towards the printer and in that direction. The new implementations of the port allow a bidirectional communication by means of these lines.

Each one of these lines (control, state, data) can be referenced of independent way by means of a registry.

The records of the parallel port

Each record of the parallel port is pointed by an address. The parallel port has three records:

    • data record

    • status record

    • control record

The next table shows the relation that exists between the physical lines of the connector of the PC and the Records.

General table of the parallel port

DB25

Signal

Record

Type

data

Sense

1

Control 0 C0- Output

Low

Inverted

2

Data 0 D0 Output

high

direct

3

Data 1 D1 Output

high

direct

4

Data 2 D2 Output

high

direct

5

Data 3 D3 Output

high

direct

6

Data 4 D4 Output

high

direct

7

Data 5 D5 Output

high

direct

8

Data 6 D6 Output

high

direct

9

Data 7 D7 Output

high

direct

10

State 6 S6+ Input

high

direct

11

State 7 S7- Input

Low

Inverted

12

State 5 S5+ Input

high

direct

13

State 4 S4+ Input

high

direct

14

Control 1 C1- Output

Low

Inverted

15

State 3 S3+ Input

high

direct

16

Control 2 C2+ Output

high

direct

17

Control 3 C3- Output

Low

Inverted

18-25

Earth        

Notes:

A data in high  is a 1, a data in low is a 0

The input and output  are from the point of view of the PC

Scheme

The parallel port schematically, is described next. Note the connection to the ISA Bus in the left part and the records in the right part.

Parallel port Schematics

Description of the components

The parallel port originally was formed by the following components:

    • 1 Latch to handle the data record

    • 1 Buffer to control the feedback of the data record

    • 1 Buffer to handle the status record

    • 1 Latch to handle the control record

    • 1 Buffer to control the feedback of the control record

    • 1 Multiplexer to addres the ports in  ISA Bus

    • 1 bidirectional Driver to connect with ISA Bus

 

Other criteria search:
RS-232, Interfaces, Diodes, Leds, Latches, Buffers, relay, transistors, real time, temperature, sensors, Pascal, Delphi, Assembler 

 

 

Operation

Explanation of the operation in the  BIOS and the MSDOS

IBM specified base addresses for the standard parallel port (within the address space of Input/output of 80x86). The printer adapter could use the base address 3BCh, or later 378h or 278h.

BIOS (Basic Input Output System) of IBM makes in the moment of  POST (Power On Seft Test) a table in the space of the main memory (ram) for 4 base addresses of parallel port of printer, these stores like 4 bytes beginning with the memory direction 408h. During the starting, the BIOS verifies if there are parallel ports in the base addresses 3BCh, 378h, and 278h, in that order, and stores to the base address of which they have been found in consecutive positions of the table. The positions that are not used can be in 0, or as some BIOS do it, they place the direction to him of the first found port.

Some programs can ignore this table, but this is used at least by the own BIOS (by means of INT 17 of printer ES) and by the MSDOS.

The BIOS detects these ports writing AAh to the registry of data (in the ES direction Base + 0), and soon if in the registry of data AAh is ***reflxed mng. It means that there is a port.

Normally the allocation of addresses are as follows: 

Address

Name

Location

3BCh LPT1 Primary printer adapter
378h LPT2 Secondary printer adapter

 

The references to each record of the port are made of the following form:

    • Base Address (data) =base+0

    • status=base+1

    • Control=base+2

For example, if we found that the base address is 378h, then the directions of the data record, status and control will be:

    • Base (data) =378h

    • Status=379h

    • Control=37Ah

Each one of them allows to access to the following bits (described in the general table):

    • Base (data) =D0, D1, D2, D3, D4, D5, D6, D7

    • Status=S3, S4, S5, S6, S7

    • Control=C0, C1, C2, C3

    •  

 

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