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HyperTerminal Test
This means that characters are being transmitted and received through the loopback. If you don’t believe us
remove the loopback connector. This time you should only see one character each time a key is pressed.
4. Crossover cable (Null modem)
A cable from a serial port always connects to another serial port. An external modem or other device that
connects to the serial port has a serial port built into it. For modems, the cable is always straight thru: pin 2
goes to pin 2, etc. The modem is said to be DCE (Data Communications Equipment) and the computer is
said to be DTE (Data Terminal Equipment). Thus for connecting DTE-to-DCE you use straight-thru cable.
For connecting DTE-to-DTE you must use a null-modem cable (also called a crossover cable). There are
many ways to wire such cable. (see page 12 for an example of typical crossover cable)
Today the PC is usually used as DTE instead of a terminal (but real terminals may still be used this way).
The names of the pins are the same on both DTE and DCE. The words: "receive" and "transmit" are from the
"point of view" of the PC (DTE). The transmit pin from the PC transmits to the "transmit" pin of the modem
(but actually the modem is receiving the data from this pin so from the point of view of the modem it would be
a receive pin). Today, people wanted to connect DTE to DTE (for example a computer to a terminal) and
various ways were found to do this by fabricating various types of special null-modem cables. In this case
what pin connects to what pin becomes significant.