Loopback Connectors

B

Loopback connectors are designed for the testing of communication ports. They take the form of either a single plug or a port-to-port cable with some communication connections shorted (looped-back).

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Note - Loopback connectors must be wired properly and connected firmly for the Serial Port Tests to work correctly. Miswired, poorly soldered, or missing loopback connectors can cause erroneous diagnostic error messages.

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Table B-1 depicts the pin assignments for most loopback plugs and cables that may be used when testing a system.

Table B-1

Signal Description	EIA	CCITT #	RS-449 "A" "B"	DIN 8 8-pin round	DB9 9-pin	DB25 25-pin	Direction	Alpha ID
Chassis/frame ground	AA	101	1 NC	NC	NC	1	none	AA
Transmit Data (TxDa)	BA	103	4 22	3	3	2	output	BA
Receive Data (RxDa)	BB	104	6 24	5	2	3	input	BB
Request To Send (RTSa)	CA	105	7 25	6	7	4	output	CA
Clear To Send (CTSa)	CB	106	9 27	2	8	5	input	CB
Data Set Ready (DSRa)	CC	107	11 29	NC	6	6	input/ output	CC
Signal Ground (SG)	AB	102	19 NC	4	5	7	none	AB
Data Carrier Detect (DCDa)	CF	109	13 31	7	1	8	input	CF
Transmit Clock In (TRxCa)	DB	114	5 23	NC	NC	15	input	DB
Receive Clock in (RTxCa)	DD	115	8 26	8	NC	17	input	DD
Data Terminal Ready (DTRa)	CD	108	12 30	1	4	20	output	CD
External Clock Out (TRxCa)	DA	113	17 35	NC	NC	24	output	DA
Secondary Data Carrier Detect (DCDb)	SCF	122	NC NC	NC	NC	12	input	SCF
Secondary Clear to Send (CTSb)	SCB	121	NC NC	NC	NC	13	input	SCB

Table B-1 (Continued)

Signal Description	EIA	CCITT #	RS-449 "A" "B"	DIN 8 8-pin round	DB9 9-pin	DB25 25-pin	Direction	Alpha ID
Secondary Transmit Data (TxDb)	SBA	118	NC NC	NC	NC	14	output	SBA
Secondary Receive Data (RxDb)	SBB	119	NC NC	NC	NC	16	input	SBB
Secondary Request to Send (RTSb)	SCA	120	NC NC	NC	NC	19	output	SCA

Notes: NC = No connection

B.1 25-Pin RS-232 Loopback Plug

The RS-232 and RS-423 single-port loopback plug is a specially wired male DB-25 connector. It is plugged in to a serial port in the back of the system under test. The wiring is shown in Figure B-1.

Figure B-1

B.2 25-pin RS-232 Port-to-Port Loopback Cable

Use these wiring directions for 25-pin RS-232 and RS-423 port to 25-pin RS 232 and RS 423 port loopback cables (two DB-25 connections). It is plugged into a pair of serial ports in the back of the system under test. Both connectors are male. The wiring is shown in Figure B-2.

Figure B-2

B.3 8-Pin to 8-Pin Loopback Cable

Use these wiring directions for 8-pin round DIN RS-232 port to RS-423 to 8-pin round-DIN RS-232 and RS-423 port loopback cable. Both connectors are male.

Figure B-3

Pin 8, Receive clock In (DD), remains unconnected.

B.4 8-Pin Loopback Plug

Use these wiring directions for male 8-pin round-DIN RS-232 and RS-423 Single port loopback plugs.

Figure B-4

Pin 8, Receive Clock In (DD), remains unconnected.

B.5 25-pin Port A-to-Port B Loopback Plug

Use these wiring directions for a 25-pin Port A to Port B loopback plug for most systems.

Figure B-5

B.6 25-pin Port A-to-A Port B-to-B Loopback Plug

If your system has a single communication port to connect it to peripherals, use these wiring instructions for making a male 25-pin loopback plug for that communication port:

Figure B-6