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March 10th
1997

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CHAPTER 7

The Cummunications Redirector



The purpose of the communications redirector is to enable you to use a server's serial ports from a client machine. This means that you may use the server's modem, without the need for dedicated network communications-software running on the server machine.

The communications redirector works by intercepting the PC BIOS INTERRUPT 14H calls on the client machine, and redirecting them to the server machine. The resulting data or status is returned to the client machine.

The communications redirector program (COMREDIR.COM) only works with communications-software that use the PC BIOS INTERRUPT 14H calls. Any software that accesses the serial port directly (through the I/O port address) cannot be used to control the server's serial port devices.

The COMREDIR.COM program must be loaded on both client, and server, machines.

Loading COMREDIR.COM

To load the communications redirector on the server or client machine, type the following at the DOS command line:

cd\dosnet

comredir

You can connect to the server's serial port device using the NET USE command, or the Station Manager. The server's serial port must have been previously shared using the NET SHARE command, or through the Station Manager.

Sharing a modem

To share a modem on the server, type the following at the server's DOS command line:

NET SHARE MODEM=COM1

Connecting to a modem

To connect to the server's shared modem from the client machine, type the following at the client's DOS command line:

NET USE COM1 \\SERVER1\MODEM

Both the NET SHARE and NET USE commands are explained in detail in Chapter 3 "THE NET COMMAND".

Once connected, you may use your communications software as normal, accessing the server's serial port device as a local serial port device.

The Communications Menus

The communication redirector's menus are accessed through the DOS-NET Main Menu using the network pop-up keys. These are normally CTRL+ALT+Right Shift keys. From the DOS-NET Main Menu, select COM Port Redirection.

The following is a step by step guide to most of the features in the communication redirector's menus.

The following examples assume that you have connected to the server's serial port (known by the network name "MODEM") using the NET USE command or Station Manager.

Initialising serial ports

The following example initialises the network modem to 2400 baud, no parity, 8 data bits, and 1 stop bit. This is normally expressed as 2400, N, 8, 1.

comredirTo initialise a serial port, select Initialise Port.


comredirNext, you will be prompted to select the port you wish to initialise. This menu shows which ports are local, and which ports are connected to a network server's serial port.


comredirNext, select the baud rate (speed) that your software requires. If you are using a modem, you must select the same baud rate that the remote-site modem is using. For the best performance, use the fastest common speed available to both modems.


comredirSelect the parity required by your communications protocol. This is normally none for Data communications, and odd or even for text-based communications.


comredirSelect the number of data bits required by your communications protocol. This is normally 8 for data communications, and 7 for text-based communications.


comredirSelect the number of stop bits required by your communications protocol. This is normally 1 for data communications, and 2 for text-based communications.

Viewing serial port status

comredirYou can view the serial port status in real time by selecting View Port Status.


comredirNext, you will be prompted to select the port you wish to view. This menu shows which ports are local and which ports are connected to a network server's serial port.


comredirThe serial port status is split in to three categories: Line Status, Modem Status and Modem Control.


The status menu may change to reflect the real time status of the serial port.

Flow control

Flow control is a simple form of communications handshaking that is used to control the flow of data from the transmitting machine to the receiving machine. When the receiving machine's buffers are full, that machine must save that data, without losing any additional incoming data.

The receiving machine must inform the transmitting machine to stop transmitting while the receiving machine saves the data in its buffers. When the receiving machine has saved the data, communications may then continue.

This handshaking is called Flow Control. It gives the receiving machine control over the flow of data.


comredirFlow control may be used by selecting Flow Control.


comredirSelect the port with which you wish to use flow control. The adjacent menu shows which ports are local, and which ports are connected to a network server's serial port.


comredirNext, you will be prompted to select the flow control you require.


There are two types of flow control CTS/RTS known as hardware flow control, and XON/XOFF known as software flow control. You may use any combination of the flow control methods. You can also disable flow control by selecting None.

Modem commands

The communications redirector includes a comprehensive set of modem commands. These commands are only available if the ModemCmds= parameter in the DOSNET.INI file is set to 1. The modem commands work on Hayes-compatible modems only.

comredirTo use the modem commands, select Modem Commands.


comredirYou will then be prompted to select the port that is connected to the modem. The adjacent menu shows which ports are local, and which ports are connected to a network server's serial port.


comredirThe modem commands available are listed in the Modem Commands menu. These commands are supported by all Hayes-compatible modems.

This menu allows you to pop-up over your existing software, and issue commands to the network modem from your client machine.

To use a command, simply use the arrow keys to move the bar cursor onto the command, and press the Enter key.

Normally, these commands are invoked using the Hayes AT command set from your terminal emulation software.


comredirThe phone-book option allows you to auto-dial any number in the phone book list. The phone book's data file is called PHONE.TXT and is located in C:\DOSNET sub directory. You can add names and numbers to the phone editing PHONE.TXT with a DOS text editor.

To auto-dial a number, select that name and number and press the Enter key.


comredirNext, select the call type, Voice or Data. If voice mode is selected the number is simply dialled. If data mode is selected, the number is dialled and data communications are established between the modems.


comredirWhile the dialling is in process, the status screen is displayed as shown. This shows the name and number being called, and the call type.

If the call is a voice call, you will be advised to pick up the phone then press any key. This informs the modem to transfer control from the modem to your phone set.

If the call is a data connection call, the chatter box is displayed as soon as communications are established.


comredirThe chatter box is split in two. Whatever you type is displayed in the bottom half, and whatever the remote user types is displayed in the top half.

Unlike terminal emulation software, the chatter box does not transmit any data until you press the Enter key. This allows you to correct mistakes before any text is transmitted.

To return to the Modem Commands menu, press the Escape key.



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