Serial & Parallel Diagnostic Loopback Plugs

By David D. Scribner

Revision 1.01

One of the many tools a PC service and repair technician keeps in their toolbox is a set of loopback plugs (wrap plugs) for diagnosing problems with RS-232 Serial and Parallel ports. A problem arises when you may have to use various software diagnostic utilities, many which in turn have their own style of loopback plug configurations. This can be easily resolved by using crimp-type D-Subminiature connectors and two-part connector hoods that screw together, and which can be found at your local electronic parts store for just a few dollars.

The crimp-type connectors attach to standard 22-gauge stranded wire, and can be easily punched back out of their housing position with a blunt object such as the tip of a closed pair of needle nose pliers when a configuration change is needed. You will likely still need to utilize your soldering skills, however, as several of the configurations require multi-pin connections.

My suggestion for other PC repair technicians out there (and what I do myself), would be to construct a set of loopback plugs for the diagnostic utilities you use most. Then, for those times when you need to use a different diagnostic package, keep extra pin configuration wire-sets in your toolbox so you can quickly reconfigure your loopback plugs for use with that vendor’s software.

This document outlines pin-out configurations used by those various diagnostic utilities, as well as the pin definitions for the serial and parallel ports used on IBM-compatible PCs. Although there are many vendor configurations included (as well as some “generic” loopback plug configurations collected through the years), some vendors such as the manufacturer of the PC-Doctor diagnostic utility make available loopback plugs for USB, game ports and even audio ports. Should you know of the wiring for those plugs, or know of a vendor’s pin-out configuration not listed here, I would certainly welcome the information and will expand this document to include those configurations as collected. My email address is dscribner @ bigfoot.com, and I look forward to adding the information to this list for others to use!

Any trademarks or registered trademarks mentioned belong to their respective owners.

RS-232 Serial Loopback Plugs (DB-9 Pin)

IBM-PC RS-232 Serial (COM) Port DB-9 Pin-outs

DB-9 PIN (Male) FUNCTION ABBREVIATION

1 <----------------------------- Data Carrier Detect DCD or CD

2 <----------------------------- Receive Data RXD or RD or RX

3 -----------------------------> Transmit Data TXD or TD or TX

4 -----------------------------> Data Terminal Ready DTR

5 ------------------------------ Signal Ground GND

6 <----------------------------- Data Set Ready DSR

7 -----------------------------> Request To Send RTS

8 <----------------------------- Clear To Send CTS

9 <----------------------------- Ring Indicator RI

Transmitted and receive data are referenced from the data device and not the modem.

DB-9 Female Serial Connector Port ( face-view, as seen on PC )

+-------------------------+

\ 5 4 3 2 1 /

\ /

\ 9 8 7 6 /

+-----------------+

Serial Port Loopback (9-pin – IBM)

Name	Pin	Pin	Name

DCD	1	7, 8	RTS, CTS

RXD	2	3	TXD

DTR	4	6, 9	DSR, RI

Serial Port Loopback (9-pin – Norton)

Name	Pin	Pin	Name

DCD	1	4, 6, 9	DTR, DSR, RI

RXD	2	3	TXD

RTS	7	8	CTS

Serial Port Loopback (9-pin – PassMark and PC-Doctor)

Name	Pin	Pin	Name

DCD	1	4, 6	DTR, DSR

RXD	2	3	TXD

RTS	7	8	CTS

Note: PC-Doctor can also support other vendor specific wrap plugs, a list of which can be obtained through the Test Options menu.

Serial Port Loopback (9-pin)

Name	Pin	Pin	Name

DCD	1	4, 6	DTR, DSR

RXD	2	3	TXD

RTS	7	8, 9	CTS, RI

Serial Port Loopback (9-pin)

Name	Pin	Pin	Name

DCD	1	6, 9	DSR, RI

RXD	2	3	TXD

DTR	4	6	DSR

RTS	7	8	CTS

Serial Port Loopback (9-pin – another version)

Name	Pin	Pin	Name

DCD	1	4, 6	DTR, DSR

RXD	2	3	TXD

RTS	7	8	CTS

RS-232 Serial Loopback Plugs (DB-25 Pin)

IBM-PC RS-232 Serial (COM) Port DB-25 Pin-outs

DB-25 PIN (Male) FUNCTION ABBREVIATION

1 ----------------------------- Chassis Ground CG or GND

2 ----------------------------> Transmit Data TXD or TD or TX

3 <---------------------------- Receive Data RXD or RD or RX

4 ----------------------------> Request To Send RTS

5 <---------------------------- Clear To Send CTS

6 <---------------------------- Data Set Ready DSR

7 ----------------------------- Signal Ground SG or GND

8 <---------------------------- Data Carrier Detect DCD or CD

9 ----------------------------> Transmit+ (Current Loop) TD+

11 ----------------------------> Transmit- (Current Loop) TD-

18 <---------------------------- Receive+ (Current Loop) RD+

20 ----------------------------> Data Terminal Ready DTR

22 <---------------------------- Ring Indicator RI

25 <---------------------------- Receive- (Current Loop) RD-

NOTE: Current loop technology was supported in the PC and XT interfaces. Current Loop was discontinued when the AT interface was introduced. Transmit and receive data are referenced from the data device, not the modem.

DB-25 Female Serial Connector Port ( face-view, as seen on PC )

+----------------------------------------------------------+

\ 13 12 11 10 9 8 7 6 5 4 3 2 1 /

\ /

\ 25 24 23 22 21 20 19 18 17 16 15 14 /

+--------------------------------------------------+

Serial Port Loopback (25-pin – IBM)

Name	Pin	Pin	Name

GND	1	7	GND

TXD	2	3	RXD

RTS	4	5, 8	CTS, DCD

DSR	6	11, 20, 22	TD, DTR, RI

TCK	15	17, 23	RCK, CHCI

RD+	18	25	RD-

Note: An old-style IBM loopback plug used a 3.9 K-Ohms resistor connected between the pin 6 and pins 11 and 22 leads. This loopback plug was used to test the IBM SDLC- and BSC-Adapters, which are not that common anymore, but if you run across one at least you can configure a plug to test it with!

Serial Port Loopback (25-pin – Norton)

Name	Pin	Pin	Name

TXD	2	3	RXD

RTS	4	5	CTS

DSR	6	8, 20, 22	DCD, DTR, RI

Serial Port Loopback (25-pin – PassMark)

Name	Pin	Pin	Name

TXD	2	3	RXD

RTS	4	5	CTS

DSR	6	8, 20	DCD, DTR

Serial Port Loopback (25-pin – PC-Doctor)

Name	Pin	Pin	Name

TXD	2	3	RXD

RTS	4	5	CTS

DSR	6	8, 20	DCD, DTR

Note: PC-Doctor can also support other vendor specific wrap plugs, a list of which can be obtained through the Test Options menu.

Serial Port Loopback (25-pin – yet another version)

Name	Pin	Pin	Name

TXD	2	3	RXD

RTS	4	5, 22	CTS, RI

DSR	6	8, 20	DCD, DTR

SERIAL PORT LOOPBACK DIAGNOSTIC TESTING RULES

When the diagnostic asserts RTS (output) it then tests for the presence of CTS and Ring Indicator (input). If CTS and RI are detected the RTS driver and CTS/RI receivers are considered operational. When DTR is asserted (output) the diagnostic tests for the presence of CD and DSR (input). If CD/DSR are detected the DTR driver and CD/DSR receivers are considered operational. Data is transmitted and received on the data lines and the data is compared in the diagnostic buffer. If any status's are not detected an error message is displayed.

Parallel Loopback Plugs (DB-25 Pin)

IBM-PC Parallel Printer Port DB-25 Pin-outs

DB-25 Male DB-25 Female

Pin Signal

1 -------------------------------> - Strobe*

2 -------------------------------> + Data 0 (least significant)

3 -------------------------------> + Data 1

4 -------------------------------> + Data 2

5 -------------------------------> + Data 3

6 -------------------------------> + Data 4

7 -------------------------------> + Data 5

8 -------------------------------> + Data 6

9 -------------------------------> + Data 7

10 <------------------------------- - Acknowledge*

11 <------------------------------- + Busy

12 <------------------------------- + Paper End

13 <------------------------------- + Select

14 -------------------------------> - Auto Feed*

15 <------------------------------- - Error*

16 -------------------------------> - Initialize Printer*

17 -------------------------------> - Select In

18 - 25 <-------------------------- - Ground (Data Return)

Note: Signals marked with an asterisk (*) denote an active low signal.

DB-25 Male Parallel Connector Port ( face-view, as seen on PC )

+----------------------------------------------------------+

\ 1 2 3 4 5 6 7 8 9 10 11 12 13 /

\ /

\ 14 15 16 17 18 19 20 21 22 23 24 25 /

+--------------------------------------------------+

Parallel Port Loopback (IBM)

Name	Pin	Pin	Name

Strobe*	1	13	Select

Data Bit 0	2	15	Error*

Acknowledge*	10	16	Initialize*

Busy	11	17	Select In

Parallel Port Loopback (Norton and PassMark)

Name	Pin	Pin	Name

Data Bit 0	2	15	Error*

Data Bit 1	3	13	Select

Data Bit 2	4	12	Paper Out

Data Bit 3	5	10	Acknowledge*

Data Bit 4	6	11	Busy

Parallel Port Loopback (AMI, CheckIt and PC-Doctor)

Name	Pin	Pin	Name

Strobe*	1	13	Select

Data Bit 0	2	15	Error*

Acknowledge*	10	16	Initialize*

Busy	11	17	Select In

Paper Out	12	14	Auto Feed*

Note: PC-Doctor can also support other vendor specific wrap plugs, a list of which can be obtained through the Test Options menu.

Parallel Port Loopback (Micro2000)

Name	Pin	Pin	Name

Strobe*	1	13	Select

Acknowledge*	10	16	Initialize*

Busy	11	17	Select In

Paper Out	12	14, 15	Auto Feed*, Error*

Parallel Port Loopback (yet another version)

Name	Pin	Pin	Name

Strobe*	1	13	Select*

Acknowledge*	10	16	Initialize*

Busy	11	17	Select In

Paper Out	12	20	Ground

Note: This parallel loopback version actually works very well to “spoof” your PC into thinking a parallel printer is attached, since the Paper Out and Error pins are not used. This is especially beneficial for older DOS-based programs that are “hard-coded” to recognize only the LPT1: port for printing. Although this works for those programs running under DOS, or in Windows 95/98SE, it will not work for those programs that do not support legacy devices in DOS-mode, such as Windows Me.

Copyright © 2002-2003 David D. Scribner. All rights reserved.	Page 7

Serial-Parallel Loopback Plugs

Contains information on various serial and parallel loopback plug configurations used by many PC diagnostic programs (IBM, Norton, PC Doctor, CheckIt, etc.).

David D. Scribner

2002-04-02T20:21:00

David Scribner