How to make your own loop back plugs

Last updated: 12/December/2009

Warning: These are the pinouts of the plugs which we have made and successfully tested with our own software on our own PC's. You use this information at your own risk! It's easy to make a mistake and we take no responsibility for the potentially serious consequences.

Serial port loopback plug pinout (9 Pin)

You'll find below the pin out instructions for making your own serial port loop back plug. These plugs are designed to be plugged into the serial ports on your computer. In conjunction with software such as BurnInTest 2.2 Pro (or later) they allow the testing of the port by joining the transmit and receive pins together.

The serial port on IBM compatible PC's conform to the RS232 electrical standard and use a male DB-9 connector. (older PC's may use a DB-25). The DB-9 connector as the following 9 pins.

If you don't want to make your own plugs, you can always purchase our serial and parallel loopback plugs.

DB-9 Pin	EIA abbreviation	Description
1	DCD	Data carrier detect
2	RXD	Receive data
3	TXD	Transmit data
4	DTR	Data terminal ready
5	GND	Signal ground
6	DSR	Data set ready
7	RTS	Request to send
8	CTS	Clear to send
9	RI	Ring Indicator

To make the loop back plug the following pins need to be connected together

• Receive and transmit (2 & 3)
• Request to send and Clear to send (7 & 8)
• Data carrier detect, Data set ready and Data terminal ready (1, 6 & 4)

This diagram shows the pinout connections that need to be made. It's the rear view of the female DB-9 connector that's required for the plug. The red lines and gray dots show the connections that need to be made.

Female DB-9 - Rear view

Serial port plugs (Old style 25 Pin)

Here are the pinout connections required for the older 25 Pin serial ports. No new PC's have 25 pin serial ports so don't confuse this with the parallel port.

Female DB-25 - Serial loop back - Rear view of pin outs

Parallel port plug loopback pinouts

You'll find below the instructions for making your own parallel port loop back plug. These plugs are designed to be plugged into the parallel port on your computer. In conjunction with software such as BurnInTest4.0 Pro (or later) they allow the testing of the port by joining the transmit and receive pins together.

The parallel port on IBM compatible PC's have always used a DB-25 connector. However, over time several changes have taken place to the electrical interface with the introduction of enhanced and bi-directional parallel ports. We believe that this plug will work with all styles of parallel ports.

To make the loop back plug the following pins need to be connected together

• Data 0 and Error status  (Pin 2 & 15)
• Data 1 and Select status  (3 & 13)
• Data 2 and Paper out status  (4 & 12)
• Data 3 and Acknowledge status  (5 & 10)
• Data 4 and Busy status  (6 & 11)

This diagram shows the connections that need to be made. It's the rear view of the male DB-25 connector that's required for the plug. The red lines and gray dots show the connections that need to be made.

Male DB-25 - Parallel loop back - Rear view of pin outs

Troubleshooting home made plugs - The top 8 problems

These plugs do work! Here are the top seven reasons why your plugs might not work.

1. You have wired the plugs completely in reverse. Rather than doing it from the rear view you have done it from the front view. (This is a very very common mistake)
2. You have printed this page and your printer hasn't printed the right hand section of the page and so you have forgotten to wire up the right side side of the plug. (Try decreasing the left and right page margins in Internet Explorer before you print.)
3. You have done a poor job soldering the wires and shorted out some additional pins or failed to make a good termination on some links. Don't even attempt to just poke loose wires in the back of a plug. They need to be soldered.
4. You have confused the parallel plug with the old style 25 pin serial plug.
5. There is a problem in your PC. There might be some other hardware using the port, you might have selected the wrong LPT1 port in BurnInTest, you might have a nonstandard PC (very rare) or your PC may be legitimately faulty.
6. You might have the parallel port disabled in BIOS.
7. The parallel port might not be set to be bi-directional in BIOS. Do not use SPP or PS/2 mode. But rather use EPP or ECP mode in BIOS.
8. There is a compatibility problem with the testing software you are using. We have only tested these plugs with our BurnInTest product.

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