25 pin Dsub PC connector
LPT Parallel port; Computer side
The Personal Computer [PC] Parallel Port Connector Pin-Out for the
Centronics Standard Parallel Port [SPP] is listed below.
This connector is also used and redefined as the IEEE-1284 -A [connector].
This topic covers the IEEE-1284-A standard as used to replace the Centronics interface, which was obsolete.
However the electrical interface described here is also out-dated. In other words IEEE-1284 is out-dated.
The Centronics parallel cable has a maximum run out to 12 feet, using a
36-pin champ connector at the printer and a 25-pin D-Sub connector on the
Host [PC] side.
The pinout for the 25-pin D-Sub connector is shown below, which is used by both the Centronics and IEEE-1284A interfaces.
The IEEE-1284 Bus cable which replaced
the Centronics bus now has a maximum run out to 25 feet. The function and pinout
between the two buses differ.
The Centronics interface is an 8-bit
[parallel] unidirectional bus.
The 36-pin centronics Champ connector
pinout used on the printer is listed on the Centronics
Connector PinOut page, or the IEEE-1284-B Connector page.
There is no defined standard for the Centronics interface, timing varied between printers from different manufacturers, as did drivers, receivers and termination values. The maximum possible transfer rate is 150kbps, but typical values were 10kbps. The electrical interface used TTL logic levels. The ICs used to drive the data lines used 74LS374, while the control lines used 7405 ICs. Resistor pull-ups for the open collector lines were 4.7k ohm to +5 volts, but could be any value because there was no specification.
The IBM PS/2 series of computer [also
obsolete] added bi-direction to the port.
The Centronics [Printer side] and original Parallel port [Computer side] interfaces are obsolete,
replaced by the IEEE-1284 Bus.
The Original IBM PC Parallel Port Pin-Out or LPT port as it was also known as, was replaced by the
IEEE-1284 cable.
The IEEE-1284 specification defines the 25 Pin D-Sub as the 'A' connector.
The IEEE-1284 'A' cable is backwards compatible with the IBM Personal
Computer [PC] Parallel Port cable and has the same pin out listed here.
The IEEE-1284 'A' cable is defined in the standard and is a better made
cable, while the original parallel port cable had no specification.
| Pin # | Pin name | Pin Description and Function |
|---|---|---|
| 1 | /STROBE | Strobe |
| 2 | D0 | Data Bit 0 |
| 3 | D1 | Data Bit 1 |
| 4 | D2 | Data Bit 2 |
| 5 | D3 | Data Bit 3 |
| 6 | D4 | Data Bit 4 |
| 7 | D5 | Data Bit 5 |
| 8 | D6 | Data Bit 6 |
| 9 | D7 | Data Bit 7 |
| 10 | /ACK | Acknowledge |
| 11 | BUSY | Busy |
| 12 | PE | Paper End |
| 13 | SEL | Select |
| 14 | /AUTOFD | Autofeed |
| 15 | /ERROR | Error |
| 16 | /INIT | Initialize |
| 17 | /SELIN | Select In |
| 18 | GND | Strobe Ground |
| 19 | GND | Data bit 1 and 2 Ground |
| 20 | GND | Data bit 3 and 4 Ground |
| 21 | GND | Data bit 5 and 6 Ground |
| 22 | GND | Data bit 7 and 8 Ground |
| 23 | GND | Busy and Fault Ground |
| 24 | GND | Paper out, Select, and Acknowledge Ground |
| 25 | GND | Auto-Feed, Select input and Initialize Ground |
Personal Computer [PC] Parallel printer bus specification defines
a Point-to-Point asynchronous bi-directional interface. Devices may be
either 1284 compatible {the older parallel port devices} or 1284
compliant. The maximum recommended length for a printer cable is 25 feet.
Centronics parallel cables run out to 12 feet. The IEEE1284 cable
replaced the 'Centronics' cable, the Centronics cable is obsolete.
IEEE1284 operates in five different modes: [The connector or cable does not change
with each different mode, but the function of some of the pins do]
Compatibility mode; Centronics type operation (PC to Peripheral),
providing the original (required) control signaling bits. These bits
include 8 data lines, a Strobe, a Busy, an Acknowledge, a Select, Paper
Empty, Fault, Initialize Printer, Select Printer, and a Auto Feed line.
Compatibility mode is the basic mode of operation, asychronous, byte wide
operation with a transfer rate of between 50kBps to 150kBps.
Nibble mode; 4 bit data bus (Peripheral to PC), 8 bit data bus (PC
to Peripheral); supporting uni-directional printer interfaces. This
provides an interface which operates full speed forward and half speed in
reverse. The transfer rate is between 50kBps to 150kBps
Byte mode; 8 bit bi-directional bus. Normal port operation. The
transfer rate is between 50kBps to 150kBps
ECP; Extended Capabilities Port. Allowed the PC to send 32 bit
data to the port, than letting the port divide up the data into four 8
byte messages, improving system (PC) operation. Transfer speeds are ten
times faster then the previous modes.
EPP; Enhanced Parallel Port; Allows high-speed transfers of bytes
in either direction. EPP is used with real time controlled peripherals.
EPP transfer times are the same as ECP transfer rates.
| Pin | Compatibility mode | Nibble mode | Byte mode | EPP mode | ECP mode |
| 1 | nStrobe | HostClk | HostClk | nWrite | HostClk |
| 2 | Data 1 | Data 1 | Data 1 | AD1 | Data 1 |
| 3 | Data 2 | Data 2 | Data 2 | AD2 | Data 2 |
| 4 | Data 3 | Data 3 | Data 3 | AD3 | Data 3 |
| 5 | Data 4 | Data 4 | Data 4 | AD4 | Data 4 |
| 6 | Data 5 | Data 5 | Data 5 | AD5 | Data 5 |
| 7 | Data 6 | Data 6 | Data 6 | AD6 | Data 6 |
| 8 | Data 7 | Data 7 | Data 7 | AD7 | Data 7 |
| 9 | Data 8 | Data 8 | Data 8 | AD8 | Data8 |
| 10 | nAck | PtrClk | PtrClk | Intr | PeriphClk |
| 11 | Busy | PtrBusy | PtrBusy | nWait | PeriphAck |
| 12 | PError | AckDataReq | AckDataReq | User defined 1 | nAckReverse |
| 13 | Select | Xflag | Xflag | User defined 3 | Xflag |
| 14 | nAutoFd | HostBusy | HostBusy | nDStrb | HostAck |
| 15 | nFault | nDataAvail | nDataAvail | User defined 2 | nPeriphRequest |
| 16 | nInit | nInit | nInt | nInt | nReverseRequest |
| 17 | nSelectIn | 1284 Active | 1284 Active | nAStrb | 1284 Active |
| 18 | (nStrobe)ground return | - | - | - | - |
| 19 | (Data 1 and 2)ground return | ||||
| 20 | (Data 3 and 4)ground return | ||||
| 21 | (Data 5 and 6)ground return | ||||
| 22 | (Data 7 and 8)ground return | ||||
| 23 | Pins 11 and 15 ground return | ||||
| 24 | Pins 10, 12, and 13 ground return | ||||
| 25 | Pins 14, 16, and 17 ground return |
The 25 pin D-sub connector is usually color-coded purple, even though there is normally only one 25 pin connector on a PC.
Which implies that no color coding is needed, but still works for a pictorial how-to manual.
Return to the IEEE-1284 bus page.
Centronics Connector
Pinout .... Parallel Port Connector
Pinout .... 1284-B Cable Pinout
.... 1284-C Cable Pinout
IEEE1284-A to 1284-B Adaptor
Pinout .. IEEE1284-A to 1284-C Adaptor
Pinout .. IEEE1284-B to 1284-C Adaptor
Pinout
Navigation: Engineering Home > Interface Buses > Cabled Interface Standards > IEEE-1284 Standard > A-Cable.
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