IEEE 1284-A Connector Pin-Out

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.

IEEE-1284 'A' Connector, 25 Pin D-Sub PinOut
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

25 pin D-sub Female connector, Plug
25 pin Dsub PC connector

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.

IEEE-1284 'A' Connector, Signals in all Modes
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

25 pin D-sub PC connector symbol
25 pin Dsub PC connector

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

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Modified 3/05/12
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