Document revision date: 19 July 1999

Once you decide which serial communications port to use, either on a host or a terminal server, you need to determine the connectors and the pinouts for the port and how to wire the modem to the port. Refer to the documentation for the modem and for the port; also see Section 7.7.2.

7.7.2 Setting Up Modems

Follow these steps to set up modems:

1. Determine connections and wiring pinouts.
   The connector and pinout determine the specific wiring adapters and cables you need to connect the modem to the port. To determine the pinout and the connector on the modem, and the pinout and connector on the port you are connecting the modem to, refer to the modem and the port documentation.
   Two common pinouts found on the EIA-232 DB25 connection are shown in Table 7-4.

   Table 7-4 Common Pinouts on the EIA-232 DB23 Connection

   Pinout	Description
   Data Terminal Equipment (DTE)	Transmit information through pin 2, and receive information through pin 3, among other standardized pin assignments.
   Data Communications Equipment (DCE)	Transmit information through pin 3, and receive information through pin 2, among other EIA-232 pin assignments.

   Straight-Through and Cross-Over Wiring
   
   Descriptions of straight-through and cross-over wiring follow:

   • DCE devices communicate straight-through with DTE devices: the transmit pin on each end of the cable is wired to the corresponding receive pin on the other end. Pin 2 on one cable is connected to pin 2 on the other cable, and pin 3 on one cable is connected to pin 3 on the other cable.
   • Equipment wired with a DCE pinout requires a cross-over to communicate with another connector wired DCE; pins 2 and 3 on one cable are connected to pins 3 and 2 on the other cable, respectively. A cross-over is required in certain situations, because two transmit pins or two receive pins cannot be wired together. As a specific example, you need a cross-over to wire two DTE devices together, or to wire two DCE devices together.
     A cable with cross-over wiring is sometimes referred to as a null modem cable, because a null modem cable of an appropriate length could logically replace all components of a modem-based communications connection; that is, it could replace the local serial cable, the local modem, the intervening telephone circuit, the remote modem, and the remote serial cable.
   
   Table 7-5 describes the most common connectors used to wire a modem.

   Table 7-5 Connectors

   Connector1	Description
   DB9	A 9-pin connector, containing a row of four pins, and a row of five pins. The DB9 can have the EIA-574 commonly used on PC systems or an older standard connection used on MicroVAX consoles.
   DB25	A 25-pin connector, with a row of twelve pins and a row of thirteen pins. The DB25 typically uses the EIA-232 pinout and can be wired as Data Terminal Equipment (DTE) or as Data Communications Equipment (DCE).
   MMJ	A 6-pin modular jack, which uses DEC-423 signaling, commonly referred to as DECconnect wiring. DECconnect wiring greatly simplifies wiring devices, as one need consider only the appropriate adapter for the device connection; the associated BC16E cabling is wired consistently.

   ---

   ¹All connectors in this table are available in both male and female genders.
   

   ---

   
   The pinouts and applications for the common connectors are shown in Table 7-6.

   Table 7-6 Connector Applications

   Connector and Pinout	Example	Adapter1
   A DB9 9-pin connector with an EIA-574 PC-compatible pinout	The DB9 connectors found on most PC, AlphaStation, and AlphaServer systems	Use the H8571-J or compatible MMJ adapter.
   A DB9 9-pin connector that predates the EIA-574 pinout	The console connector on various MicroVAX systems uses a pinout that predates the EIA-574 pinout	Use the H8575-B or compatible MMJ adapter.
   A DB25 25-pin connector with the EIA-232 wiring	The communications ports on many terminals	Use the appropriate adapters from the following list, 2 or contact a Compaq sales representative or Compaq reseller for information on adapters not listed below: H8575-A DB25 female to MMJ adapter, straight-through 3 H8571-C DB25 male to MMJ adapter, cross-over 4 H8575-E DB25 male to MMJ, straight-through 3 A compatible adapter
   An 8-pin DIN (round) connector		Use the H8584-AB or compatible MMJ adapter.
   A Modified Modular Jack (MMJ) DECconnect socket

   ---

   ¹This table contains only a subset of the DECconnect adapters available. The adapters listed in this table might not be suitable for your particular application requirements; additional DECconnect adapters are available from Compaq.
   ²The genders listed are those of the connector on the adapter.
   ³Straight-through indicates that the EIA-232 Transmit Data signal is wired to the DEC-423 Transmit Data signal, and so on.
   ⁴Cross-over indicates the EIA-232 Transmit Data is connected to the DEC-423 Receive Data, and vice versa, and that DTR and DSR are similarly connected.
   

   ---

   
   If your application does not use one of the serial wiring connections shown in the table, you need to determine the specific requirements of the device, as well as the specific pinout. You also need to determine the cabling appropriate for the application. Contact your hardware support organization, your Compaq support representative, or your local Compaq reseller.

   MMJ Accessories
   
   Table 7-7 lists order numbers and descriptions of some DECconnect accessories available from Compaq.

   Table 7-7 DECconnect Accessories

   Order Number	Description
   BC16E-02 BC16E-10 BC16E-25 BC16E-50 BC16E-A0	DEC-423 (based on EIA-423) MMJ office cable, available in various lengths.
   H8571-C	25-pin male EIA-232 to DEC-423 DECconnect adapter.
   H8571-E	DEC-423 DECconnect 25-pin adapter with jack screws.
   H8571-J	9-pin MMJ adapter. Used with the PC-compatible EIA-574 DB9 wiring.
   H8572-00	MMJ cable extender. Allows the direct connection of two BC16E cables.
   H8575-A	Female 25-pin DEC-423 DECconnect MMJ to EIA-232 general-purpose adapter.
   H8575-B	Female 9-pin DEC-423 DECconnect to printer adapter. Also used with the DB9 wiring found on some MicroVAX console ports.
   H8584-AB	8-pin DIN to DEC-423 DECconnect adapter. Most commonly used with various Apple computers.

2. Choose a type of modem control.
   As part of connecting a modem to a device, you can add wires to the host port and the modem. These wires are used to pass signals called the modem control signals.
   When you connect to a local terminal for dial out, modem control is not particularly significant: either the modem is wired or configured to ignore modem control, or the wiring is set up to pass the modem control signals from the terminal to the modem.
   When you connect a modem to a computer, modem control is far more significant, because the host uses the modem control signals to direct the modem to accept incoming telephone calls. The modem control signals also enable the modem to signal the host that a call has been received or that a call has ended. These signals allow the host and the modem to take the appropriate actions for a particular event.

   Note In addition to their use by modems, modem control signals are also often used to communicate device status between the host and other serial devices such as serial printers. Various serial printers use modem control signals as modems do: to indicate to the host that the printer is powered up and ready to accept output, or that the printer is powered down or otherwise unable to process output.

   
   Table 7-8 contains descriptions of types of modem control that devices can support.

   Table 7-8 Types of Modem Control That Devices Support

   Type of Modem Control	Description
   No modem control	The host and the modem cannot intercommunicate the status of the host or the modem. It is possible to use a modem on this port; however, this type of port is not recommended for a modem. Without modem control, the modem cannot signal the host that the telephone call has been disconnected and that the host must take appropriate action: suspend or log out the associated user process. (See Step 5 for the associated security implications.) Furthermore, without modem control, you must set or wire the modem so that it always answers incoming calls, because the modem cannot know if the host is able to respond. (This too has security and modem control implications.)
   Limited modem control	The host and the modem can intercommunicate and can take actions based on the status of the other device. Limited modem control is the best choice for most applications.
   Full modem control	The host and the modem can intercommunicate and can pass an extensive amount of control and status information. Both the host and the modem can take actions based on the status of the other device. Limited modem control, which has similar capabilities, has largely superseded this configuration. Limited modem control also requires fewer wires on the connection, making it the more economical choice.

   
   Refer to the device documentation to determine the type of modem control signal that the device and modem support. This determines the number of wires and the wiring connections needed for communications. The following examples show types of modem control and the wires they require:

   • DECconnect supports limited modem control, which requires two of the six wires in the DECconnect cabling. The other four wires are used for the following purposes:
     • Transmitting data
     • Receiving data
     • The transmit ground
     • The receive ground
   • Full modem control requires more than two wires dedicated to the modem control signaling.
   • Devices that do not support modem control require no wires dedicated to modem signaling.
   
   With modem commands or custom-wired cabling, you can force a modem to operate with a device that does not support modem control. However, this is not recommended for general use on a host system, because this wiring can potentially result in security problems.

3. Determine the command set used by the modem.
   The command set includes the commands used to request that the modem place a telephone call, the telephone number to be called, and the commands used to configure the modem.
   Examples of command sets follow:
   • AT command set:

     ATDT phone-number

     
     where:

     • AT indicates "attention"---to get the attention of the modem
     • DT indicates "dial tone"; (PT would indicate "pulse tone").
   • DMCL command set:

     Ctrl/B [Return] Ready DIAL T phone-number

     
     where:

     • T represents "tone"; (P would represent "pulse").
     • phone-number represents the phone number you are dialing.
   
   The command set is used to communicate with the modem to request that the modem perform some action, such as dialing a telephone number and connecting to a remote modem. You can enter direct modem commands at a terminal directly connected to a modem, or you can communicate indirectly with the modem using DCL commands such as SET HOST/DTE.
4. Configure the port.
   After wiring the modem to the connector on the OpenVMS computer or DECserver, you must configure the port to recognize and properly operate the modem, and to enable autobaud speed detection.

   Note The autobaud operation detects the speed---the baud rate---of the communications. Including the /AUTOBAUD qualifier is not required; however, if autobaud detection is disabled, you must configure both the host terminal or DECserver port, and the modem, for the same baud rate.

   
   The commands you give depend on whether you are using an OpenVMS host system or a DECserver:
   • On an OpenVMS host system, execute the following command interactively, and also place this command in the system-wide startup file, SYS$MANAGER:SYSTARTUP_VMS.COM:

     $ SET TERMINAL /MODEM /AUTOBAUD /PERMANENT TTAO:

     where TTA0: is the name of the terminal device the modem is wired to.
     This command requires privileges.

   • On a DECserver, configure the port using the following commands:

     DECserver> SET PORT n MODEM ENABLE DECserver> SET PORT n FLOW CONTROL XON ENABLE DECserver> SET PORT n AUTOBAUD ENABLE

     where n is the port number.
     The commands enable the modem, XON, and autobaud. These commands require privileges on the DECserver.

5. Ensure security with your modems.
   Dial-in lines allow remote, unauthorized users access to your system. You need to maintain consistent security and good system and user password management to keep your system secure from unauthorized users.
   The following list contains some ways to increase security on your system:
   • You can configure a DECserver with a password to prevent a modem from accessing any other feature. This password prevents an unauthorized user from accessing or seeing any information about the local network configuration until after the user enters the password. You can enable this password on specific ports.
   • With OpenVMS, you can establish a system-wide password requiring the user to specify a password before the system prompts for a password. This additional password helps reduce the security risk caused by users with poor passwords. You can enable a system-wide password on specific host ports.
   • With OpenVMS, you can establish minimum password lengths, and you can enable system-generated passwords. These measures can help reduce the security risk caused by users with poor passwords.
   • Always use and configure some form of modem control. Without modem control, a telephone connection that is disconnected for any reason might be left logged into the host, and a subsequent modem caller will receive the logged-in session without specifying a password. Also, without modem control, the host cannot request that a modem session be dropped when certain system events such as a process logout occur.
   
   These and other techniques for protecting your system from unauthorized access are discussed in detail in the OpenVMS Guide to System Security.

In troubleshooting any serial communications problems, particularly those problems with a modem, attempt to isolate the problem as much as possible, testing one component, wire, or device at a time.

Table 7-9 contains some general troubleshooting suggestions, but it is not a complete list. Basic serial communications test equipment such as a serial-line break-out box, can often help you locate communications and wiring problems. For further assistance, contact your local hardware support organization.

Table 7-9 Troubleshooting Communications Problems

Problem	Considerations
Modem does not answer	Check that the telephone number being called is correct.
	Check that the modem has power.
	Check that the host system or device has power and is operating.
	If possible, directly connect a standard terminal in place of the modem, and test the operation directly.
	Check that the host modem control signals are present, and correctly wired.
	Check that the host device is configured correctly for a modem by using a SET TERMINAL, SET PORT, or other appropriate host command.
	Check the wiring. Look for a broken, miswired, or disconnected wire.
	Look for a disconnected connector or a broken, missing, or bent connector pin.
Telephone malfunction	Using a standard telephone handset, test that voice calls can be established on the telephone line.
	Is static or other interference noticeable on the telephone line?
No modem indicator lights	Check the power connection.
	Check to see that the modem is turned on.
	Check to see that the modem has passed applicable self-tests.
	Try swapping the modem for another device.
No response, or garbled response to typing	Check that the modem status lights indicate received data on the transmit line, and transmitted data on the receive line. This can point to the miswiring of the transmitted and received data. You can wire serial cables and adapters straight-through or with a cross-over.
	Check for crossed signals.
	Check for incorrect speed detection. Autobaud detection sometimes sets the speed incorrectly. On lines that are not enabled for autobaud detection, check that the line is set for the correct speed. On ports that support it, check the speeds for both the transmitted and the received data.
	Make sure that the port has autobaud enabled, and that the port and the modem are configured for the same data rate.
	Check for interference or a disconnection in the wiring.
	Check the wiring for any problems.
	Check for any adjacent wiring, power, or video signals that might interfere with the serial communications.

7.8 Managing Printers

To manage printers attached to your system, perform the following tasks:

Task	For More Information
Set printer characteristics	Section 7.8.1
Spool printers	Section 7.8.2.1
Despool printers	Section 7.8.2.2
Test spooling of printers	Section 7.8.2.3

	privacy and legal statement
6017PRO_027.HTML