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Appendix B - RAS Cabling

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Most ISA and EISA computers have one of the following serial port connectors:

  • 25-pin male "D-shell" connectors

  • 9-pin male connectors

Most—but not all—off-the-shelf cables will work with your modems. Some cables do not have all the pins connected as shown in the following tables. When purchasing cables, tell your dealer exactly what you need, and provide the information in these tables to be sure you have the correct match.

Note Do not use the 9-to-25-pin converters that come with most mouse hardware, because some of them do not carry modem signals.

25-Pin Cabling

As Table B.1 shows, pins 1 through 8 on the serial port connector are wired to their counterparts on the modem connector. Ribbon cables usually have all 25 pins wired straight across, but they can cause interference to TVs, radios, and VCRs. To prevent this problem, use shielded RS-232 cable.

25-pin serial port connector

25-pin modem connector

Signal

1

1

Ground

2

2

Transmit Data

3

3

Receive Data

4

4

Request to Send

5

5

Clear to Send

6

6

Data Set Ready

7

7

Signal Ground

8

8

Carrier Detect

20

20

Data Terminal Ready

9-Pin Cabling

The following table shows how to connect a 9-pin serial port connector on a computer to a 25-pin connector on a modem. Again, if you buy an off-the-shelf cable, be sure all pins are connected as shown in Table B.2.

9-pin serial port connector

25-pin modem connector

Signal

1

8

Carrier Detect

2

3

Receive Data

3

2

Transmit Data

4

20

Data Terminal Ready

5

7

Signal Ground

6

6

Data Set Ready

7

4

Request to Send

8

5

Clear to Send

9

22

Ring Indicator (optional)

Note Some modems have the Data Set Ready (DSR) signal physically tied to the Data Carrier Detect (DCD) signal. Some 1200-bps modems and other 2400-bps modems have dip switches default to this setting as well. As a result, if such a modem loses power while listening for a call, the Remote Access server cannot detect the condition because the DSR will not change as it does with other modems.

Serial Cabling Requirements

The remote Access Service requires the following pins on the RS-232 cable:

Rx

Receive

Tx

Transmit

CTS

Clear To Send

RTS

Ready To Send

DTR

Data Terminal Ready

DSR

Data Set Ready

DCD

Data Carrier Detected

Caution All the pins listed above must be present. The Remote Access Service does not work if any of the seven pins is missing. If any pins are not present and working, the Remote Access Service reports a hardware error.

Null Modem Cabling

If you are using a null modem to make a direct serial connection between two computers, your cable must be wired as shown in tables B.3 and B.4.

Remote host serial port connector

Calling system serial port connector

Signal

3

2

Transmit Data

2

3

Receive Data

7

8

Request to Send

8

7

Clear to Send

6, 1

4

Data Set Ready and Carrier Detect

5

5

Signal Ground

4

6, 1

Data Terminal Ready

Remote host serial port connector

Calling system serial port connector

Signal

2

3

Transmit Data

3

2

Receive Data

4

5

Request to Send

5

4

Clear to Send

6, 8

20

Data Set Ready and Carrier Detect

7

7

Signal Ground

20

6, 8

Data Terminal Ready

Off-the-shelf null modem cables might be improperly wired. Be sure to tell your dealer that your null modem cables must be wired as shown in Table B.4.

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