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Using your PC compatible computer as a dumb terminal


Some electronic devices including many embedded microprocessor based devices can be configured or operated using only a "dumb terminal". The expression "dumb" means a basic terminal with just a screen, keyboard, and serial port connector, and without any of the "smarts" that a computer has. However, with PC compatible computers being much more common than terminals, and having the same screen, keyboard, and serial port, it is often desirable to use a computer for this purpose.

Most PC compatible computers can be used as a terminal if there is a serial port connector and its associated software COM port that is not being used. All that is needed is the proper cable to connect from the serial port connector to the equipment with which you are communicating, and software to emulate a terminal.

A little background on PC serial ports:

The original IBM PC compatible system software was capable of utilizing just two COM ports, COM1 and COM2. Plug-in boards that used serial port hardware, like dual serial port boards and modems, could be selected with jumpers plugs on the boards for either of those two COM ports. This means that a physical serial port connector could be associated with either COM port.

Serial port boards that allowed you to connect a cable had male 25 pin "D" connectors. Later, as a space saving measure, many manufacturers changed to 9 pin "D" connectors for one or more serial ports, since most of the signals on the 25 pin connectors were not being used anyway. Inexpensive cable adapters are available from any computer or electronics store to change to and from the 9 pin standard to the 25 pin standard.

Now, many PC's have system software that can utilize four COM ports. Most plug-in boards, as well as on-board serial port hardware, can now be set up as any of the four COM ports, and many of them do not require jumper plugs at all. Some computer systems and their associated plug-in boards, called "plug-and-play", can determine what boards are installed and configure their COM ports automatically.

Finding a spare serial port

A spare physical serial port can be found by finding a 25 pin or 9 pin male "D" connector with nothing connected to it. Knowing which software COM port is associated with this connector can be determined by finding out what serial devices are already using the other COM ports, and knowing how some of these devices respond can help do this.

Many PC's use one of the physical serial ports for the mouse, and you may be able to tell which one if the serial connector is labeled on your computer. The mouse may also be connected though a special connector and still be associated with a particular COM port. The mouse COM port is usually set for 1200 baud, 8 bits, 1 stop bit, and no parity. The characters sent from a mouse when moved or clicked are not displayable characters. When displayed on the screen using a terminal program they will not make particular sense, and may even cause some programs to lock up. However, just observing that these characters are being sent can be useful in determining the COM port to which a mouse is connected.

Internal plug-in modems also use serial port hardware and one of the COM ports. Modems will usually work at any baud rate up to the specified speed of the modem, and will automatically determine what baud rate is being used and respond accordingly. A modem will usually echo back any characters sent to it, and this will help determine the COM port to which a modem is connected. Modem commands are fairly standard, so by sending the modem a standard "attention" command, which you can do by typing the characters A, T, and the ENTER key, you can usually get the modem to respond with an "OK".

Once a spare serial port connector has been found, and the COM port associated with it is determined as best as possible, there is a method that can be used to positively identify it. Pins 2 and 3 of either a 25 pin or 9 pin serial connector can be connected together, which connects the transmit and receive signals. This will cause any keys typed to be echoed back to the screen. This will identify the COM port by observing this echoing of characters, and then seeing it quit when the signals are no longer connected.

Common terminal emulation software

DOS:
Various shareware and commercial terminal emulation programs are available, many of which have features far beyond simple terminal emulation, for example, ProComm, a popular shareware program. Instructions for setting up the COM port, and other communication parameters will be available in the program.

Windows 3.x Terminal:
From the Program Manager window click on "Terminal" in the "Accessories" window. If "Terminal" has never been run on your computer since Windows was installed it will ask you for a "Default serial port". It will require that you choose one that is available and will then display other windows of the communications parameters it needs. If it has been run before, you will need to go to the menu bar at the top and select "Settings". Under "Terminal emulation" select "TTY Generic", under "Communications" select the "baud rate", "data bits", "stop bits", "parity", "flow control", and "connector" (COM port). If you do not know which "Flow Control" option to select, use "Hardware". You may need to change this setting later if proper communication does not occur. To quit the program go to the menu bar at the top and select "File" and then "Exit". If everything works correctly and you want to save this configuration, answer yes to the prompt and then choose a directory, then choose a name for the connection that you will remember as being used for the device with which you are communicating with which to store the configuration file. The next time you run "Terminal" you will be able to go to the menu bar and select "File" and "Open" to select this configuration file where your customized configuration is saved.

Windows 95/98 HyperTerminal:
This program will allow you to save the setup that you specify, so choose a name for the connection that you will remember as being used for the device with which you are communicating. The setup can be edited later if the selected communication parameters or COM port do not work correctly.

From the "Start" icon select "Programs" and then the "Accessories" option in the menu that follows. Select "Hyperterm" from the "Accessories" group and click on the "Hyperterm". If HyperTerminal is run without specifying a saved configuration, an initial series of screens will appear as follows. In the "Connection Description" window enter the name and select the icon. In the "Phone Number" window under the "Connect Using" option select "Direct to COM1" (2, 3, or 4). In the "Port Settings" window select the "Bits per second", "Data bits", "Parity", and "Stop bits" to match those of the device with which you are communicating. If you do not know which "Flow Control" option to select, use "Hardware". You may need to change this setting if proper communication does not occur. To quit the program go to the menu bar at the top and select "File" and then "Exit". If everything works correctly and you want to save this configuration, answer yes to the prompt. This will cause your customized configuration to appear in the HyperTerminal window as the name and icon you chose.





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